During the past decade the use of alternative fuels for diesel engine has received considerable attention. The interdependence and uncertainty of petroleum-based fuel availability and environmental issues, most notably air pollution, are among the principal forces behind the movement toward alternative sources of energy. Several alternative fuels are available, but all of them are hydrocarbon-based fuels, which cannot eliminate the net carbon emissions. One alternative is to make use of a non-carbon fuel like hydrogen. In the present investigation, hydrogen was used in a diesel engine in the dual-fuel mode with diesel as a primary fuel. Experiments were conducted to determine the optimized injection timing, injection duration, and hydrogen flow rate. From the results it is observed that the optimum timing in port injection is 5 degrees before gas exchange top dead center (BGTDC) with an injection duration of 30 degrees crank angle (CA) and in manifold injection at gas exchange top dead center (GTDC) with an injection duration of 30 degrees CA. Hydrogen flow rate was varied from 2 to 9.5 lpm with above the above-optimized conditions for both port and manifold injection. The optimized hydrogen flow rate was found to be 7.5 lpm for both port and manifold injection. Flow rates higher than 9.5 lpm shows an improvement in performance and reduction in emissions, but the onset of knock was observed; hence, the flow rate was limited to 9.5 lpm. At 75% load the brake thermal efficiency increases by 21% in port injection and 18% in manifold injection. NOx emission is reduced by 2% in port injection and 4% in manifold injection compared to diesel at full load. At full load, smoke is reduced by 45% in both port injection and manifold injection. In the entire load spectra a reduction in CO by about 50% is noticed in both port and manifold injection. Ignition delay or a delay period is found to be 11 degrees or 1.22 ms for diesel and 10 degrees or 1.11 ms in both port and manifold injection.