This paper describes with numerical studies on predicting for combustion and exhaust gas emissions in the cylinder of a diesel engine manufactured for medium duty trucks. The EGR rate is changed from 1 to 37% under the low load, engine speed 1500 rpm condition. The predicted pressure, rate of heat release and NOx have good agreements with measured ones. NO2 is formed by the conversions from NO formed by post flame reactions by oxidation in the cylinder. Soot is also tried to simulate under the same condition. However, it is difficult to predict both qualitative and quantitative soot emission values.