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Homogeneous charge compression ignition (HCCI) combustion is a hybrid concept of compression and spark ignition combustions. It is a promised solution to environmental and fuel economy concerns for internal combustion engines. In this mode of combustion, a lean premixed charge combusts simultaneously from multiple sites. Utilizing very lean mixtures, and the lack of any obvious flame propagation, considerably reduces in-cylinder NOx formation. In order to make the HCCI engine a feasible alternative to the SI and CI engines, several items must be elucidated. Control of the combustion timing is one of the most important of these items to be resolved. Combustion timing should be controlled in order that heat is released at the best time in the engine cycle. In this study, a Waukesha CFR single cylinder research engine with variable EGR was used to be operated in HCCI combustion mode fueled by natural gas and n-heptane.