This study was performed to investigate the effects of humid air on nitrogen oxide (NOx) and particulate matter (PM) emissions of a compressed natural gas (CNG) engine. A 50 horse-power, 4 cylinder, naturally aspirated CNG engine was used for this study at a research facility in the College of Engineering at California State University Long Beach. The experiments were carried out at different relative humidity levels of 30%, 45%, and 60% at different power outputs of 5 HP, 12.5 HP, 25 HP, and 37.5 HP. At each test condition, the NOx and PM emissions are collected and analyzed through a dilution tunnel, the Horiba gas analyzer, and a cyclone with Teflon air filters. For PM measurements, the Teflon filters are weighed using a micro-balance at University of California, Irvine’s biology lab, in prior and after running the experiments. Results show a decrease in NOx emission up to 91 percent and increase in PM emission by 7.4 mg with the increase in intake air humidity level.