The analysis of combustion for D. I. diesel engine with high pressure injection has been investigated using a digital image processing technique by tracing history of flame luminosity distribution. The effects of air swirl and injection pressure on turbulence intensity, turbulence scale and turbulent mixing rate were examined. As a result, it becomes clear that with high swirl ratio, combustion deteriorates because of the lower turbulent mixing rate in the later period of injection, and that with high pressure injection, combustion improvement is obtained through the increase of turbulent mixing rate at the early stage of diffusion burning.