A new Emission Control System (ECS) based on the water injection to the charge air was developed for the reduction of NOx emissions from the marine diesel engines. The system was installed into a medium speed diesel engine in a general cargo ship and tested under various speed and load conditions, in accordance with the MARPOL convention. The experimental results show that the engine performance was almost unaffected, while the system caused a large reduction of the NOx emissions. Even without optimization of the experimental conditions the reduction of NOx emissions was about 70%. The levels of carbon monoxide(CO) and carbon dioxide (CO2) were almost unchanged.