Abstract In CNG/Diesel reactivity controlled compression ignition (RCCI) engines, because of low reactivity fuel (natural gas) spread of flame in combustion chamber suffers process of combustion and also there is more unburned hydrocarbon emission in this type of engine due to flame spread, free-combustion points, penetrating gas into rings and cavities. Using hydrogen as an additive can improve combustion process, improve engine performance and reduce emissions. By this research, a computational fluid dynamic modeling in coupling with Chemkin solver has been used to investigate the effects of hydrogen as an additive in RCCI CNG/Diesel internal combustion engine. Numerical results have been validated by a single cylinder Ricardo E6/MS engine that equipped by a standard Comet MK.V pre-chamber. Three different concentrations (10%, 20% and 30% in volume) of hydrogen have been studied by this modeling. Specification of Ricardo engine have been used for CFD modeling from IVC to EVO as a close system modeling. There is a good flow structure with good swirling flow before combustion that makes fuel more suitable for starting combustion. HRR diagrams show a delay in combustion by adding hydrogen in natural gas and air. IMEP, gross work and IFCE improved by 5.01%, 5.01% and 5.04% in concentration of 30% hydrogen, respectively. There are a descend in mass fraction of UHC and CO in different hydrogen concentration that 30% H2 addition in fuel makes the amount of UHC and CO lower by 29.8% and 35.5%, correspondingly. This is in a condition that in-cylinder temperature rises about 2.9% by 30% of hydrogen adding. NOx emissions has been risen during adding H2 because of increasing temperature, this increment is about 23.1% at 30% hydrogen that can be an undesired effect of using H2 as an additive. In general, the simultaneous use of Pre-chamber and hydrogen has been able to reduce the amount of CO and UHC, but the amount of nitrogen oxide has been increased.