During the past decade, considerable efforts have been made to introduce alternative fuels for use in conventional diesel and gasoline engines. There is significant interest in adding hydrogen to a diesel engine to reduce emissions and improve efficiency. With the rapid increase in computational capabilities, computational fluid dynamics (CFD) codes have become essential tools for the design, control, and optimization of dual fuel engines. In the present study, a reduced chemical kinetics mechanism, consisting of 52 reactions and 29 chemical species for n-heptane fuel combustion, was incorporated with detailed chemical kinetics consisting of 29 reactions for hydrogen including additional nitrogen oxidation. This reaction mechanism was coupled with a 3D CFD model based on AVL FIRE software to investigate the performance and emission characteristics of a diesel engine with low amounts of hydrogen addition. The model was validated by the experimental results and then employed to examine important parameters that have significant effects on the engine performance. The simulation results showed that the variations of brake thermal efficiency, CO2, CO and NOX emissions reasonably agree with the experimental findings. NOX emissions and exhaust gas temperature increased with the rise in brake power for hydrogen-diesel mixtures. The CFD results quantified the degree of dependence of NOX emissions on the average combustion temperature. The results also quantified that CO and CO2 emissions decreased when adding hydrogen in diesel engine because the addition of hydrogen leads to reduction in the injected amount of diesel fuel that results in CO and CO2 formation.