By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a primary reference fuel (PRF) with octane rating between 0 and 100. The influences of PRF fuel's octane number on the combustion characteristics, operation range, performance, and emissions characteristics of homogeneous charge compression ignition (HCCI) engine were investigated. The experiments were carried out on a modified single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delays, the combustion rate decreases, the combustion duration prolongs, and the cylinder pressure decreases. The HCCI combustion can be controlled, then the HCCI operating range can be extended by burning different octane number fuel at different engine modes, in which engine burns low octane number fuel at small load mode and large octane number fuel at high load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load. With the increase of the PRF fuel octane number, NOx, HC and CO emissions increase, especially for HC emissions.