The performance of a natural gas fueled spark ignition engine was studied to investigate the effect of hydrogen enrichment on the engine operating range and combustion characteristics. A validated multi-dimensional CFD model coupled with detailed chemical kinetics mechanism was used to study the combustion process in Hydrogen/CNG engine. Results indicated that misfiring occurs in equivalence ratios of about 0.61, 0.48 and 0.42 for hydrogen fractions of 0%, 30% and 50% respectively. Also, combustion durations were 70, 47 and 45 degrees crank angle for hydrogen fractions of 0%, 30% and 50% respectively with same equivalence ratio (Phi = 0.625). Regarding the emission, the maximum amount of break specific NOx was detected around (Phi = 0.83 (lambda = 1.2), and this point is almost independent of the hydrogen fraction. It was found that hydrogen fraction decreases the emissions of CO, and HC and CO2. Adding 30% hydrogen reduced CO2 emission by10.2% while 50% hydrogen fraction led to 22.7% CO2 reduction. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.