The effects of the fuel-air ratio of the combustible mixture and the intake temperature on the homogeneous charge compression ignition (HCCI) combustion behavior were studied on a rapid compression machine. The experiment results indicate that with the increase of the intake temperature, the ignition timing advances, the combustion duration decreases, the maximum combustion temperature rises, the maximum heat release rate increases, the maximum pressure rise rate becomes higher and its corresponding timing advances. With the increase of the excess air ratio of the combustible mixture, the start of combustion delays, the ignition temperature increases, the maximum combustion temperature decreases, the combustion duration increases, the maximum heat release rate decreases, the maximum pressure rise rate becomes lower and its corresponding timing delays.