Abstract: To understand the effects of air humidity on diesel engine emissions, the NOx and soot emissions were calculated during diesel combustion at different humidity ration (0, 5, 10 and 20 g/kg) using KIVA-3V software. The thermal physical parameters, intake composition and intermediate production were obtained. The simulation results show that NOx decrease linearly with increasing humidity ratio. At the humidity ratio of 20 g/kg, the NOx reduction of more than 30% can be achieved. Except at heavy load, humidity has a little effect on soot. Besides N2 and O2, O and OH radicals are reduced with increasing humidity ratio. At heavy load and high humidity, the decrease of O2 and soot oxidation resulting from decreasing O2 greatly contributes to soot yield. Humidity has dual effects on soot formation.