The effect of syngas addition on the performance of a 1.6 L gasoline engine at lean condition was investigated in the paper. The syngas which produced by the onboard ethanol catalytic decomposition was mainly composed of hydrogen and carbon monoxide. A tube array reforming reactor was mounted on the engine tailpipe to produce syngas. During the test, the engine was run at 1800 rpm and a manifolds absolute pressure of 61.5 kPa. The spark timing for the maximum brake torque was adopted for all tests. The engine spark timing, injection timing and duration of the gasoline were controlled by a hybrid electronic control unit communicated with the engine original electronic control unit. The syngas volume fraction in the total intake gas was gradually increased from 0% to 1.84%. The gasoline flow rate was decreased to ensure that the global excess air ratio of the fuel-air mixture in cylinder at about 1.20. The test results confirmed that the syngas addition helped improve the indicated thermal efficiency and shorten the combustion duration. HC, NOx emissions and particle total number per cubic centimeter were reduced after the syngas addition at lean condition. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.