The main objective of this study was to investigate the effect of hydrogen addition on spark ignition (SI) engine’s performance, thermal efficiency, and emission using variable composition hydrogen/CNG mixtures. The hydrogen was used in amounts of 0%, 20%, 40% by volume fraction at each engine speed and load. Experimental analysis was performed at engine speed of 1200 rpm, load of 120 Nm corresponding BMEP = 0.24 MPa, spark timing 26 CAD BTDC, and at engine speed of 2000 rpm, load of 350 Nm corresponding BMEP = 0.71 MPa, spark timing 22 CAD BTDC. The investigation results show that increasing amounts of hydrogen volume fraction contribute to shorten ignition delay time and decrease of the combustion duration, that also affect main combustion phase. The combustion duration analysis of mass fraction burned (MFB) was presented in the article. Decrease of CO2 in the exhaust gases was observed with increase of hydrogen amounts to the engine. However, nitrogen oxides (NOX) were found to increase with hydrogen addition if spark timing was not optimized according to hydrogen’s higher burning speed.