The proposed two-zone model of diesel combustion consists of a burned zone and an unburned zone, and the thermodynamic process is independent and adiabatic in each zone except that the air is entrained from the unburned zone to the burned zone under a specified condition of the excess air ratio during the combustion period. The excess air ratio in the diffusion combustion period is assumed to be constant in the present model. The calculated time histories of the burned-zone gas temperature and the cumulative soot formation were compared with the measured time histories of the flame temperature and the KL value based on the infrared two-color method. The calculated results agree qualitatively and partly quantitatively with the experimental results except for the swirl effect. As a result, it is shown that the present two-zone model analysis is very useful and effective in evaluating the combustion process in a DI diesel engine.