Numerical modeling of soot formation and oxidation in diesel sprays was conducted using a multistep soot model coupled with reaction mechanisms for fuel oxidation and polyaromatic hydrocarbon (PAH) formation. As new combustion strategies such as low-temperature combustion emerge, the demand for accurate numerical models has also increased to predict the effects of subtle changes in the operating conditions on combustion and exhaust emissions. Accurate prediction of soot emissions from diesel engines remains challenging, particularly for low-temperature combustion conditions with massive exhaust gas recirculation. Soot emissions from the engine are highly sensitive to local temperature and chemical compositions. In this article, an n-heptane mechanism is combined with a detailed PAH mechanism to simulate diesel spray combustion and emissions formation. The mechanisms are validated against experimental data of ignition delays and flame speeds. The overall reaction mechanism consists of 68 species and 145 reactions and is used with a multistep soot model. The multistep soot model is employed to simulate the evolution of soot including inception, surface growth, coagulation, and oxidation. The soot model is validated against experimental results obtained from a constant-volume combustion chamber and a heavy-duty diesel engine. Predicted flame liftoff locations and flame structures agree well with the experimental data. The model was able to predict the characteristics and trends of soot emissions with respect to the level of exhaust gas recirculation. Further analyses of the engine simulation results were conducted for understanding the soot emission characteristics in the engine.