In this study, we investigated the relationship of indicated mean effective pressure (IMEP) for a spark ignition engine under lean combustion with the cyclic variation of mass fraction burned by measuring the energy release from the spark plug, intensity of the light emmision from the flame and the cylinder pressure at the same time. In order to minimized an error in the initial and late combustion stages of the mass fraction burned to be obtained by cylinder pressure, spark plug energy and intensity of light emission were measured. As a result, it was found that there are three main couses of cyclic variation of IMEP. These consist of the burning speed during the initial stage of combustion, variation in the total mass fraction burned, and variation of the late burning during the late expantion stroke. Thus, we determined that there is a favorable interrelatonship between the IMEPs and the corrected mass fraction burned.