A statistical measurement of cyclic variation in combustion of a spark ignition engine generated by changes of mixture conditions in the cylinder was compared with the actual measurement of variation of flow velocity and mixture ratio.From these statistical data plus further analytical study, the correlations between the combustion speed, flow velocity, and mixture ratio were shown. These correlations explain the causing reason of combustion variation-it increases with the increase of combustion period.Regarding cyclic variation of combustion, although every possible factor influencing the variation of combustion was eliminated there still remains the variation caused by the variation of mean flow velocity in the cylinder. Thus, to decrease the combustion variation, it is fundamentally necessary to raise the combustion speed and decrease the variation of mean flow velocity in the cylinder. Since the rate of increase of the combustion variation caused by the variation of flow velocity decreases when the flow velocity becomes faster, the fast swirling one directional undisturbed main stream increases the combustion speed and is accompanied by small cyclic fluctuation which produce stronger turbulence that efficiently contribute to this effect.By measuring the curve of S(dp/dt) max versus mixture ratio, it was possible to indicate whether mixture ratio or velocity variations were causing cyclic fluctuations. Also it was clarified that Tpmax was able to accurately indicate the trend of mean combustion period.