The combustion processes of a diesel engine operating at low speeds and low loads with different EGR rates were investigated. The CHEMKIN-Ⅱ chemistry solver was integrated into the KIVA 3V Release 2 code and a reduced n-heptane kinetics mechanism was employed as the surrogate of the diesel oxidization mechanism to model diesel combustion. Combined with the experimental data, simulations and analysis were performed for EGR rates sweeping from 0% to 60% while the injection timing was unchanged. The results have shown that the combustion phase is retarded obviously with high EGR rates. The combustion temperature is decreased and the combustion path avoids the high NOx formation regions. The NOx emission of the case with EGR 60% is reduced by 93.5%. But combustion path with high EGR rate does not avoid the high level soot formation regions and the diluted air is unfavorable to the oxidization of the soot. So the soot emission is much higher.