The reactivity controlled compression ignition (RCCI) combustion has the potential to simultaneously reduce the NOx and PM emissions and maintain combustion performance even when injection timing is advanced. Because intake air temperature is an important factor affecting the reactivity of fuels, it is necessary to study optimized fuel supply ratios according to the intake air temperature. Therefore, the purpose of this study was to analyze combustion and exhaust characteristics in relation to the fuel supply ratio, injection timing, and intake air temperature. In this study, ethanol was injected into an intake port; increasing the ethanol supplied ratio increased the ignition delay. Thus, the net indicated mean effective pressure (IMEPnet), compared with conventional diesel combustion, increased from 4.14 to 4.90 bar for the advanced injection timing (BTDC 27 degrees). In addition, because the combustion period was lengthened and combustion temperature lowered, the NOx emission decreased (19.1 -> 2.7 g/kWh); however, the THC (1.1 -> 2.5 g/kWh) and CO (5.2 -> 10.1 g/kWh) emissions increased. Moreover, burning an homogeneous mixture of ethanol decreased the particulate matter emission from 74 to 45 mu g/m(3). However, under high intake air temperature conditions, the effect of ethanol ratio on ignition delay was small. Therefore, the injection timing at which the maximum IMEPnet occurred was retarded. In addition, as the intake air temperature increased, the THC and CO emissions decreased and that of NOx increased.