The paper studied the effect of syngas addition on performance of a gasoline engine at lean conditions. The engine ran at 1800 rpm and a manifolds absolute pressure of 61.5 kPa. The spark timing for the maximum brake torque was adopted for each testing point. The syngas volume fraction in the total intake gas was fixed at 0% and 2.5%. The test results showed that peak cylinder pressure and indicated thermal efficiency were enhanced after the syngas enrichment. Flame development and propagation durations of the 2.5% syngas-blended engine were reduced by 7.2 and 5.7 degrees CA, compared with those of the original engine at an excess air ratio of 1.36. The coefficient of variation in the indicated mean effective pressure showed a noticeable decrease after the syngas addition. CO and NOx emissions were slightly increased with the syngas enrichment. HC emissions were first reduced and then increased after the syngas blending. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.