A computer program (DIFSIM : DIFfusion SIMulator) was written to calculate the impurity profile, specifically boron and phosphorus, due to three different diffusion processes-predeposition, drive-in in inert ambient, and drive-in in oxidizing ambient. The vacancy mechanism including Fair and Tsai``s theory for phosphorus diffusion was widely incorporated for modelling various diffusion processes. The concentration-dependent oxidation rate was also explained using the vacancy model, while the oxidation-enhanced diffusion was modelled using Watkins replacement mechanism. The simulation results using DIFSIM show a fairly good agreement with the experimental data, by adjusting some of the empirical parameters in the program. The results obtained using DIFSIM was also compared with SUPREM II, version 0-05, the mostly being-used process simulator.