The performance and emission of a CNG-DI and spark-ignition engine when a small amount of hydrogen is added to the CNG using in-situ mixing is studied. The injection timing was set to 300 degrees BTDC, the air-fuel ratio was kept at stoichiometric, and the ignition timing was adjusted to obtain the maximum brake torque. The experiments were performed at 2000, 3000, and 4000 rpm of engine speeds with each operating at WOT conditions. The introduction of a small amount of hydrogen had improved the engine performance, Brake Specific Energy Consumption (BSEC), and cylinder pressures as well as emissions at low engine speed. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.