In this study, the effects of reference fuels with different research octane numbers (RONO, RON20 and RON40) on engine performance were investigated at 40 degrees C intake air temperature and full load in a four cylinders HCCI engine. The operating range for the HCCI engine has been determined and the brake specific fuel consumption maps for each fuel have been created. Combustion characteristics were investigated at a constant engine speed of 1000 rpm. The heat release rate and in-cylinder pressure lambda values of lambda=2.15 were obtained for the engine operating in natural-aspirated mode. As the number of octane increases, the engine load increases but the operating range decreases. The lowest BSFC values were obtained at 238.7 g/kWh for RONO, 251.3 g/kWh for RON20 and 276.6 g/kWh for RON40. It was observed that there was a delay in the start of combustion as the number of octane increased. The best CA50 value was obtained 6 degrees CA from aTDC with RON40 because of the slower burning than RONO and RON20.