To achieve hydrogen power system with high performance and stable operation, cycle variation of hydrogen-fueled engine with direct injection was evaluated with several variables such as engine speed, injection timing, air-fuel equivalence ratio and spark timing. And the cause of cycle variation was analyzed by using coefficient of variation in combustion period defined in this study. The results showed that the cycle variation of hydrogen-fueled engine is mainly dependent on the early combustion period. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.