The effects of humid air on the performance of a naturally aspired three-cylinder diesel engine with low sulfur diesel fuel have been investigated. The additions of the humidity to intake air were performed with a variable steam generator using distilled water, where the relative humidity levels of the intake air were changed from the ambient conditions of 65% to 75% and 95% levels. The tests were performed at two approximate engine output brake horse powers (BHP) of 5.9, and 8.9. Results showed approximately 3.7% and 22.5% reduction in NOx emissions when the relative humidity of the air was increased from 65% (the ambient relative humidity) to 75% and 95% respectively. The addition of the humidity results in increases in the CO, CO₂, and particulate matter (PM), by approximately 3.7, 3.55, 14.9 percents at 5.9 BHP and 22, 2.8, and 9.3 percents at 8.9 BHP. There was no change in the brake specific fuel consumption (BSFC) at 5.9 BHP and about 2.7 increase in the BSFC at 8.9 BHP. Results indicate that for both mobile and stationary diesel engines, humid air system is a viable option for attaining significant reductions in NOx emissions.