The effect of port water injection on NOx formation was examined theoretically as well as experimentally. In the experiment, water was injected into each suction port of a four cylinder turbocharged DI diesel engine using gasoline injectors. The exhaust NOx was reduced significantly by port water injection, and about 50 % reduction in NOx concentration was attained under various engine operation conditions by injecting the amount of water of 0.03 kg per unit kg of dry air. Comparing the experimental results and the analytical ones based on the two-zone model developed by the authors, it is shown that the NOx reduction rate due to port water injection is dependent on the equivalent absolute humidity of the injected water and is independent on engine load, fuel injection timing and water injection timing. In other word, it becomes clear that the reduction rate of NOx in the case of port water injection is almost independent on the heat release pattern, and the NOx formation is suppressed by a decrease in temperature of the burned gas, which results from an increase in specific heat of the in-cylinder gas due to humidification.