In comparison to the extensive experimental studies, numerical studies of a diesel/methanol dual fuel (DMDF) engine are very limited. In this work, an improved KIVA-3V code was coupled with the CHEMKIN solver to model combustion and emission of a DMDF engine. First, a chemical reaction mechanism including 65 reactions and 43 species was developed and validated with premixed flame species profiles. Second, new engine experiments at varied loads in both diesel and DMDF modes were performed to further validate the present model. Results show that the model predicts the engine combustion and emission in two modes well. At last, parameter studies on diesel injection timings and exhaust gas recirculation (EGR) rates were performed. Results show that the ignition delay time in DMDF mode is longer than that in diesel mode. Methanol supplied through the intake port significantly reduces soot emissions and slightly increases NOx emissions at different injection timings. In two modes, soot emissions are both formed mainly in the bowl bottom, bowl lip near the combustion chamber center, and squish area. With the increasing EGR rate, NOx emissions are reduced significantly and soot emissions increase significantly in two modes. Combustion characteristics in DMDF mode are more sensitive to EGR than in diesel mode.