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An experimental study was carried out to investigate the characteristics of cycle-by-cycle variations of combustion in a spark ignition engine. Cylinder pressure - crank angle histories of the 240 consecutive cycles were measured in the single cylinder by using a personal computer (IBM-PC AT) controlled data acquisition system which is developed for engine tests. From these data, a heat release analysis was performed for cycles and the averaged. It was observed that characteristics of the pressure and combustion rate related parameters varies with operating conditions such as air fuel ratio, ignition timing, and so on. Variation in early flame stage is the major cause for for the cycle-by-cycle variation of combustion. It was also noticed that the variation in early flame stage does not influence the mean effective pressure significantly in case of a rapid burn operation.