The soot formation inside a diesel engine was studied by the analytical model. The soot is formed from gaseous carbon atoms by the phase change under the saturation condition. This soot model is implemented into the KIVA-3V code. From the results of the model, it is found that the fuel rich core of spray inside a flame is the main source of soot. The effect of injection timing is investigated by the soot model. When the injection timing is advanced, the formation of soot is suppressed because of high saturation pressure. The soot formation is increased when the injection timing is retarded mainly due to the decreased soot oxidation at low cylinder temperature.