Natural gas has been considered as one promising alternative fuel for internal combustion (IC) engines to meet strict engine emission regulations and reduce the dependence on petroleum oil. Although compressed natural gas (CNG) intake manifold injection has been successfully applied into spark ignition (SI) engines in the past decade, natural gas direct injection compression ignition (DICI) engine with new injection system is being pursued to improve engine performance. Gas jet behaves significantly different from liquid fuels, so the better understanding of the effects of gas jet on fuel distribution and mixing process is essential for combustion and emission optimization. The present work is aimed to gain further insight into the characteristics of low pressure gas jet. An experimental gas jet investigation has been successfully conducted using tracer-based planar laser-induced fluorescence (PLIF) technique. For safety reason, nitrogen (N₂) was instead of CNG in this study. Vapor acetone was selected as a tracer. The fourth harmonic of an Nd:YAG laser (266 nm) was used to excite the acetone, and the fluorescence signal was detected by a high-resolution charge coupled device (CCD) camera with an image intensifier. A series of instantaneous images captured in different delays after start of injection (ASOI) was used to study the time evolution and spatial distribution of fuel-air mixing processes and fuel concentration fields. The effects of gas injection pressure were investigated to characterize the mixture formation and the jet macroscopic structure. To further understand the gas jet characteristics, quasi-steady gas jets were studied under different injection pressures. Moreover, the different characteristics between transient jet and quasi-steady jet were compared and analyzed.