The effect of the addition of hydrogen (H-2) on the combustion process and nitric oxide (NO) formation in a H-2-diesel dual fuel engine was numerically investigated. The model developed using AVL FIRE as a platform was validated against the cylinder pressure and heat release rate measured with the addition of up to 6% (vol.) H-2 into the intake mixture of a heavy-duty diesel engine with exhaust gas recirculation (EGR). The validated model was applied to further explore the effect of the addition of 6%-18% (vol.) H-2 on the combustion process and formation of NO in H-2-diesel dual fuel engines. When the engine was at N = 1200 rpm and 70% load, the simulation results showed that the addition of H-2 prolonged ignition delay, enhanced premixed combustion, and promoted diffusion combustion of the diesel fuel. The maximum peak cylinder pressure was observed with addition of 12% (vol.) H-2. In comparison, the maximum peak heat release rate was observed with the addition of 16% (vol.) H-2. The addition of H-2 was a crucial factor dominating the increased NO emissions. Meanwhile, the addition of H-2 reduced soot emissions substantially, which may be due to the reduced diesel fuel burned each cycle. Furthermore, proper combination of adding H-2 with EGR can improve combustion performance and reduce NO emissions. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.