An experimental investigation on the influence of different hydrogen fractions and EGR rates on the performance and emissions of a spark-ignition engine was conducted. The results show that large EGR introduction decreases the engine power output. However, hydrogen addition can increase the power output at large EGR operation. Effective thermal efficiency shows an increasing trend at small EGR rate and a decreasing trend with further increase of EGR rate. In the case of small EGR rate, effective thermal efficiency is decreased with the increase of hydrogen fraction; while in the case of large EGR rate, thermal efficiency is increased with increasing of hydrogen fraction. For a specified hydrogen fraction, NO(x) concentration is decreased with the increase of EGR rate and this effectiveness becomes more obviously at high hydrogen fraction. HC emission is increased with the increase of EGR rate and it decreases with the increase of hydrogen fraction. CO and CO(2) emissions show little variations with EGR rate, but they decrease with the increase of hydrogen fraction. The study shows that natural gas-hydrogen blend combining with EGR can realize high-efficiency and low-emission spark-ignition engine. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.