Numerical and experimental investigations of the effect of a humid air system on nitrogen oxides (NOx) and Particulate Matter (PM) emissions of a compressed natural gas (CNG) engine have been performed. For the numerical modeling, non-premixed combustion in a single cylinder was simulated using the presumed probability density function combustion model. Simulations were performed for dry as well as humid intake air at 10%, 15%, and 30% relative humidity. Numerical results have shown 40% reduction in NO emission at 10% relative humidity, when compared with emissions of dry intake air. With 15% and 30% relative humidity levels, NO emission were reduced by 65% and 93% respectively. For experimental investigations, a General Motors inline 4 cylinders, naturally aspirated engine with a maximum rated horsepower (HP) of 50.8 for natural gas fuel was used. The engine was connected to a water-cycled dynamometer. NOx emission was measured by a Horiba portable emission analyzer model 250 and exhaust PM was monitored by a TSI DustTrak model 8520. The experiments were carried out at five different HP of approximately 5, 12.5, 25. 37.5, and 45, and three humidity levels of ambient (45%), 60%, and 75%. Results show for each additional 15% increase in relative humidity, there was a nearly 25% reduction in NOx emission at all HPs investigated, which is consistent with the corresponding drop rate between 15% and 30% relative humidity from the numerical simulations. The PM (aerosol) emission increases with the addition of relative humidity, especially at low HPs. With increase in HP, the PM augmentation is reduced significantly and at 45 HP , the change in PM emissions at 60% and 75% relative humidity, when compared with the ambient condition, were -4% and 3% respectively. Additional experiments are underway for accurate assessment of PM emission, using a Cyclone with Teflon filter.