The effects of pulsed water injection at the intake port of a modern port fuel injection gasoline engine were investigated. A port water injection system was developed and the water injector was installed on the intake runner of the single cylinder motorcycle engine at a location upstream of the fuel injector. The results show that with a water-gasoline injection ratio of 1, more than 80% of NOx emission can be removed. The trade-off was a 25% reduction in torque output at 4000 rpm and 20% throttle opening; however, the decrease on torque can be controlled to be within 5% by reducing water-gasoline mass ratios to less than 0.6. We also performed NOx emission modeling using one-dimensional gas dynamics code with extended Zeldovich mechanism, and consistent results were found between numerical prediction and experimental measurements. The port water injection approach appears to be an effective means for reducing NOx emission from a gasoline engine at low speed and high load conditions without largely sacrificing the performances on torque output and unburned hydrocarbon emissions.