Exciplex-based planar fluorescence technique was applied for two-dimensional visualization of the fuel spray including the region close to the nozzle tip. A spray doped with small amount of naphthalene and TMPD was discharged from a diesel nozzle into a pressurized gaseous nitrogen inside the test chamber installed with glass windows. The fuel spray was also allowed to evaporate in high temperature gaseous environments produced by combustion of the homogeneous mixture of methane and air in the test chamber. Photographs of the temporally frozen two dimensional image of the fuel spray were processed using an image analyzer. The image in the longitudinal cross section passing through the center axis of the spray demonstrated that the high density portion of liquid fuel appeared almost periodically downstream and that the axial distance between the neighboring high density portion increased with an increase in the downstream distance. Also observed was the existence of the surface wave which initiated to appear at a location submillimeter downstream the nozzle tip.The nature of the spray has a dominant effect on the subsequent processes of mixture formation, ignition, combustion and formation of undesirable pollutants inside the combustion chamber of diesel engines. An understanding of the physics of spray behavior is necessary to provide accurate quantitative predictions which can be interfaced with combustion models and aerodynamics in internal combustion engines. Especially, the knowledge of internal structure of a spray near the nozzle exit is a important to explore the liquid jet breakup and atomization processes.In the view of the importance of the behavior of fuel spray inside the combustion chamber of a diesel engine, a number of research work have been carried out on this topic. Most of early works concentrated on measurements of the spray angle, penetration rate and droplet size. Hoorays and Kadota (1)* measured the Sauter mean diameter in a diesel spray by using liquid immersion technique. Kamimoto et al. (2) analyzed the image of a shadow picture of a spray based on the incident light extinction principle, and measured the Sauter mean diameter. Reitz and Bracco (3) performed the photographic study on the mechanism of atomization and measured the spray angle at various operating conditions. Takahashi et al. (4) measured the spray tip penetration and the spray angle of non evaporating and evaporating sprays by using high speed photography. Probe measurements have been carried out to determine the extent of the intact core within the spray by Chehroudi et al. (5).The laser diagnostics have been recently used in order to obtain the spatially resolved information of the droplet velocity and diameter. Wu et al. (6) measured the fuel droplet velocity within the non-evaporating, steady, diesel type spray by laser Doppler velocimetry. Laforgia et al. (7) and Emerson et al. (8) measured the droplet size distribution by using the Marvern Laser Diffraction Particle Sizer which is based on the analysis of the diffraction pattern produced by the droplets. Arcoumanis et al. (9) also used the same System to obtain information of the average droplet size, and measured droplet axial mean and rms velocities using LDV. Koo and Martin (10) made the simultaneous measurement of droplet sizes and velocities by using the Aerometrics Phase Doppler/Particle Analyzer, and determined the correlation between droplet size and velocity in a transient diesel spray. Maly et al. (11) developed the optical diagnostic system with microsecond time resolution to measure the mean droplet size and number of droplets simultaneously.Most of the optical diagnostic techniques mentioned above were applied for single point measurements. Data from the single point measurements are often hard to interpret in terms of the whole structure of the spray. An alternate approach is to use laser sheet illumination to make the two-dimensional visualization of a spray. The laser sheet method has been also used for the visualization of soot distribution in the combustion chamber (12) and of the species concentration and temperature in turbulent flames (13). Felton et al. (14) used the laser sheet method to visualize liquid fuel of the spray injected in the combustion chamber of an internal combustion engine. Melton (15), and Melton and Verdieck (16) have shown that spectrally separated fluorescence emissions from liquid and vapor phases can be obtained by an exciplex visualization system based on TMPD and naphthalene. Bardsley et al. (17,18) have applied the exciplex fluorescence technique for the two-dimensional visualization of the liquid and vapor phase fuel distributions in the combustion chamber. Hodges et al. (19) applied the exciplex fluorescence method and the ensemble light scattering/polarization-ratio method in order to provide the two-dimensional data within the spray. They obtained the semi-quantitative information about gas and liquid concentrations and the ensemble averaged droplet size and concentration. Azetsu et al. (20) have performed the two-dimensional visualization of non-evaporating sprays and reported the existence of the branch-like structure in the spray.The main objective of present work is to obtain more detailed information about the internal structure of a diesel spray. The focus of this paper is understanding of the internal structure in the vicinity of the nozzle tip, which is important to explore the physical mechanism for breakup process and atomization of a spray (21). This problem is also important in the modelling and computer simulation of sprays (22).The exciplex-based planar fluorescence technique was applied for two-dimensional visualization of the fuel spray in high pressure gaseous environments. The high magnification photography was used to observe the internal structure of the spray in detail. Photographs of the temporally frozen image of the fuel spray were processed using the image analyzer.The qualitative results of liquid concentration within the spray were obtained for both of the isothermal (non-evaporating) and evaporating sprays. The effects of the injection pressure and the ambient pressure on the internal structure were described. The planar Mie scattering method and shadow photography were also applied to visualize the fuel spray.