The influence of inlet pressure (P-in) and octane numbers on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine was experimentally investigated. The tests were carried out in a modified four-cylinder direct injection diesel engine. Four fuels with different research octane number (RON) were used during the experiments: 90-RON, 93-RON, and 97-RON primary reference fuel (PRF) blend and a commercial gasoline, 94.1-RON(G). The inlet pressure conditions were set to give 0.1, 0.15, and 0.2 MPa of absolute pressure. The results indicate that, with the increase of inlet pressure, the start of combustion (SOC) advances and the cylinder pressure increases. The effects of the PRF octane number on SOC are weakened as the inlet pressure increased. However, the difference of SOC between gasoline and PRF is enlarged with the increase of the inlet pressure. The successful HCCI operating range is extended to the upper and lower load as the inlet pressure increased. The maximum achievable load of gasoline is higher than that of PRF with the cases of supercharging. The HC and NOx emissions of the HCCI engine decrease when supercharging is employed, while CO emissions increase remarkably. The PRF octane number has little effect on HC, CO, and NO., emissions when supercharging is employed. Nevertheless, the HC and CO emissions of gasoline are higher than those of PRF with supercharging.