Abstract Reactivity controlled compression ignition engines have been proven to have better performance comparing with other methods of low temperature combustion strategies. Using various fuels can have different outputs according to the chemical products and reactions as well as heating values. In this study, a numerical model beside experimental data as validation is used to investigate the effects of using additive on combustion characteristics of natural gas/dimethyl-ether and natural-gas/diesel Reactivity controlled compression ignition engines. Hydrogen is used as additive with 3, 6 and 9 percentage of heating value in the fuel mixture. Results show higher indicated mean effective pressure in all cases of using diesel as high reactivity fuel. In addition, investigation on the combustion characteristics shows that natural gas/diesel cases have advanced start of combustion and combustion phasing, while burn duration in natural gas/dimethyl-ether cases is higher than natural gas/diesel cases. By adding hydrogen species, it is seen that hydrogen has more effect on the start of combustion of natural gas/dimethyl-ether case where adding 9% hydrogen, advanced the start of combustion about 2 crank angle degree while this amount for natural gas/diesel case is about 0.3. In all cases of using diesel as high reactivity fuel, temperature is higher than dimethyl-ether used cases, which causes to produce more nitrogen oxides; for example, in 9% hydrogen addition, natural gas/diesel mode produced 0.54 g/kW/h nitrogen oxide more than natural gas/dimethyl-ether mode. Based on achieved results, carbon monoxide emission in natural gas/diesel mode is lower than 2 g/kW/h for all cases where this emission is higher than 8 g/kW/h in each case of natural gas/dimethyl-ether. This condition was also occurred for unburned hydrocarbons emissions, where this emission is higher than 11 g/kW/h for natural gas/dimethyl-ether fueling case while it is lower than 2 g/kW/h for natural gas/diesel mode. Quantitatively comparison shows that hydrogen addition is more effective on natural gas/dimethyl-ether reactivity controlled compression ignition mode. According to dimethyl-ether breaking up process, start of injection provides a time to decomposition of dimethyl-ether to its products, especially Methane. Based on numerical results, more than 10% of dimethyl-ether is broken up before start of combustion that represented importance of start of injection in natural gas/dimethyl-ether case.