The experiments to determine the effect of fuel-injection timings on engine characteristics and emissions of a DI engine fueled with NG-hydrogen blends (0, 3, 5 and 8) at various engine speeds were conducted. Three injection timings namely 120°, 180° and 300° CA BTDC with a wide open throttle at relative air-fuel ratio, λ = 1.0 were selected. The ignition advance angle was fixed at 30° CA BTDC, while the injection pressure was fixed at 1.4 MPa for all the cases. The tests were firstly performed at low engine speed of 2000 rpm to determine the engine characteristics and emissions. The results showed that the engine performance (e.g. Brake Torque, Brake Power and BMEP), the cylinder pressure and the heat release have the highest values at the injection timing of 180° CA BTDC, followed by the 300° CA BTDC and the 120° CA BTDC. The NOx emission was found to be highest at the injection timing of 180° CA BTDC. The THC and CO emissions were found to decrease while the CO2 emission increased with the advancement in the injection timing. The addition of a small amount of hydrogen to the natural gas was found to increase the engine performance, enhance combustion and reduce emissions for any selected injection timings. Secondly, the tests were carried out at variable engine speeds (i.e. 2000 rpm-4000 rpm) in order to further investigate the engine performance. The injection timings of 180° and 300° CA BTDC with CNG-H2 blends were only selected for comparisons. The injection timing of the 300° CA BTDC was discovered to yield better engine performance as compared to the 180° CA BTDC injection timing after a cutoff engine speed of approximately 2500 rpm. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.