Hydrogen internal combustion engine is considered as a suitable pathway to hydrogen economy before fuel cell technologies become more mature and cost effective. In this study, hydrogen combustion in a spark ignited direct injection (DI) engine that was optimized for natural gas application was studied. The performance characteristics such as the torque, power, BSFC were measured and analyzed for engine speeds of 1800 to 4000rpm. The combustion parameters such as the heat release were also analyzed and compared to natural gas operations. In general, the combustion of hydrogen was found to be less dependent on engine speed in comparison to compressed natural gas (CNG). It was found that low speed operation (engine speed below 2000rpm), the hydrogen combustion gave higher power output compared to CNG, owing to the more complete combustion. At higher speed, however, the performance dropped significantly due to the retarded ignition timing required to suppress pre-ignition. Copyright © 2009 SAE International.