An experimental study on the effect of hydrogen fraction and EGR rate on the combustion characteristics of a spark-ignition engine fueled with natural gas-hydrogen blends was investigated. The results show that flame development duration, rapid combustion duration and total combustion duration are increased with the increase of EGR rate and decreased with the increase of hydrogen fraction in the blends. Hydrogen addition shows larger influence on flame development duration than that on rapid combustion duration. The coefficient of variation of the indicated mean effective pressure increases with the increase of EGR rate. And hydrogen addition into natural gas decreases the coefficient of variation of the indicated mean effective pressure, and this effectiveness becomes more obviously at high EGR rate. Engine fueled with natural gas-hydrogen blends combining with proper EGR rate can realize the stable low temperature combustion in gas engine. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.