A new model to describe diesel combustion process has been developed. In this model diesel combustion field is divided into two zones, premixing and combustion. Turbulent mixing process is described by the stochastic approach in each zone separately. Comparison of calculations with experimental results showed that this model can predict the entire course of heat release and nitrogen-oxide formation precisely, under wide-spread conditions. Two-dimensional flame temperature distributions in the combustion field by the two color method were compared with simulation results. Both the measured and the calculated flame temperature distributions showed good agreements with each other.In the diesel combustion process, the injected fuel mixes with air entrained inside the spray. The mixture is thus formed, and ignites at several points. Random expansion of flamelets accelerates both mixing and combustion. Following this, fairly moderate diffusion combustion proceeds. Recent research works indicate that, the fuel droplet injected into the cylinder evaporates in a very short interval, especially in the case of high pressure injection and small nozzle hole diameter combinations (1)*. This suggests that the mixing process plays a dominant role in diesel combustion. The stochastic approach to describe the turbulent mixing process for diesel combustion was initially introduced by Ikegami and one of the authors (2) (3). This model expresses the heterogeneous nature of temperature and fuel concentration distributions by probability density functions (PDF), and describes the change of PDF for the more homogeneous states. Originally this model treated the combustion field after ignition as a macroscopically homogeneous (have the same PDF) state. On the basis of the results of combustion observation experiments, this original model has been modified.