Abstract The effects of hydrogen enrichment in a naturally aspirated spark ignition engine fuelled with compressed natural gas were investigated. A mixer that can control the enrichment fraction and assure homogeneity of hydrogen in CNG was used in the experiments. Hydrogen enrichment mitigates some of the inherent problem with natural gas engine by improving combustion due to its higher heating value, wider flammability limits, lower ignition energy and faster flame speed. The work aimed at analyzing effect of hydrogen enrichment between 10% and 30% by volume the on performance and emission at full load conditions across various engine speeds. Results show that hydrogen enrichment leads to gain in volumetric efficiency resulting in more power. High enrichment fraction is more favorable at low speed with respect to output power, while exhaust emissions of CO, CO2 and unburned hydrocarbons decrease with increasing fraction. The optimal engine speed to benefit from this system is 3000 rpm which showed good balance between output power and emissions. The optimal enrichment fraction will depend largely on the engine speed, loads and the intended balance between power and emissions.