The objective of this study is to numerically investigate the effect of the premixed ethanol fuel ratio based on the same total heating value on the distribution characteristics of equivalence ratio and NOX formation in a reactivity controlled compression ignition (RCCI) engine under early injection conditions. This work is also focused on supplementing the decrease in the combustion performance according to the advance of injection timing in premixed charge compression ignition (PCCI) engine. To implement the RCCI combustion, a gasoline direct injection (GDI) injector was installed at the premixed chamber for the ethanol injection, which was located before the intake port. The supplied ethanol amount into the cylinder was varied from 0% to 40% based on the total heating value of the fuel. In the simulation, the ethanol into the cylinder through the injection in the premixed chamber was assumed to have evaporated completely, and it was expressed by changing the initial air composition in the cylinder. To express physical and chemical reactions in simulation, the combustion model was employed ECFM-3Z (The 3-zones Extended coherent flame model) and the break-up model was applied to KH-RT (Kelvin-Helmholtz Rayleigh-Taylor) model. The results were analyzed in terms of an ignition delay, combustion duration, distribution characteristics of equivalence ratio and exhaust emissions. It was found that the values of ISNO were reduced when the premixed ethanol fuel ratio was increased because the intake air temperature was decreased due to the evaporation latent heat of ethanol injected into the premixed chamber. The low intake air temperature decreased the combustion temperature, which reduced the formation amount of NO. In addition, the rich fuel-air mixture region which is a high combustion temperature was decreased because the amount of diesel fuel was decreased by increasing the premixed ethanol fuel ratio so, the formed NO amount was reduced.