The hydrogen-diesel dual fuel combustion was investigated in direct injection (DI) diesel engine. The investigation presented in this paper preferred hydrogen as a long-term renewable and least polluting fuel among various alternative fuels for internal combustion (IC) engines. In the current study a diesel engine is made to run using hydrogen in dual fuel mode with diesel, where hydrogen is introduced into the intake manifold using an LPG-CNG injector and pilot diesel is injected using diesel injectors. The hydrogen energy contents of the total fuel were varied from 0%, 11%, 17%, 30% and 42% (the 0% hydrogen energy content represents neat diesel fuel), were experienced at (1500 +/- 10) rpm of invariable engine speed and 5.2 kW of consistent indicated power. The test results showed the improvement in brake thermal efficiency (BTHE) of the engine, reduction in brake specific energy consumption (BSEC) with an increasing hydrogen energy fraction. Furthermore, indicated specific CO, CO2 and smoke emissions decrease with an increasing percentage of hydrogen energy content. Conversely, indicated specific NOx emissions increases with increase in hydrogen content. In addition to that, it was also observed that there was a sharp increase in peak in-cylinder pressure and the peak heat release rate values with the increasing hydrogen rate. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.