The effect of fuel and operating conditions on the combustion process, load range, and exhaust emissions of a homogeneous charge compression ignition (HCCI) engine was investigated in a modified four-cylinder direct-injection diesel engine through an experimental study. Six fuels were used during the experiments: two primary reference fuels (PRF), two mixtures of PRF and ethanol, and two commercial unleaded gasoline fuels. All research octane numbers (RON) of these fuels are over 90. Six operating conditions were considered, including different intake temperatures (T-in), intake pressures (p(in)), and engine speeds (n). Experimental results indicate that autoignition of gasoline is earliest under low Pin but the PRF is earliest under high Pin. That is, the effect of fuel properties on the HCCI combustion process depends upon the operating conditions. It is beneficial to extend the load range with the sensitive fuels and suitable control strategies. For a sensitive fuel, a higher Tin is needed to extend the load range toward light load, while a lower Tin and higher Pin is needed to extend the load range toward high load. In addition, the octane index (OI) does not show a correlation with autoignition, HC and CO emissions, and load range when the mixtures of PRF and ethanol are used in some operating conditions.