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This study analyzes the mass, number density and size of the soot formed and oxidized inside a direct injection diesel engine. The predictions were made using the modified KIVA-3V code with the Foster soot model for the different engine operating variables. The multi-step reactions in the Foster model were converted into a system of ordinary differential equations, which was solved using the VODE ODE solver. The computed soot emissions were well compared with the engine-out data obtained from experiments. The change of the soot size during the expansion stroke was investigated. The surface growth, coagulation and oxidation of soot particles continuously changed size distribution. The sizes of soot particles at EVO timing were predicted to range from 1 nm to 50 nm. Retarded SOI timing increased the portion of diffusion combustion.