Natural gas is an attractive alternative fuel for internal combustion engines. Homogeneous charge compression ignition (HCCI) combustion is considered to be one of the most promising measures for increasing thermal efficiency and reducing emissions, but it is difficult to control and stabilize its ignition and combustion processes.This paper describes an experimental study of natural gas combustion utilizing two types of ignition assistance. Spark assistance, which is used for conventional spark ignition (SI) engines, and pilot diesel injection, hereinafter called diesel pilot, which generates multiple ignition points by using a small injection of diesel that accounts for 2% of the total heat release for the cycle. The performance of these two approaches was compared with respect to various combustion characteristics when burning homogeneous natural gas mixtures at a high compression ratio. A rapid compression and expansion machine (RCEM) was utilized to reproduce and visualize the inherent combustion processes. Inspection of combustion movies recorded using a high-speed video camera revealed that premixed flame is initiated at the spark plug or around the diesel spray, propagates and induces HCCI in the end gas region. Spark assistance can thus improve thermal efficiency when using rapid HCCI combustion. In contrast, diesel pilot enhances combustion velocity, which increases thermal efficiency maintaining a moderate rate of HCCI combustion. Our results also suggest that HCCI with diesel pilot is more robust and stable than spark-assisted HCCI combustion under leaner mixture condition.