It is well known that combustion stability under idle and part-load conditions directly affect fuel economy and exhaust emission. In practice, there have been a lot of studies so that a significant improvement in combustion stability has been achieved in this research field. However, applying published results to the development process of mass production engine, there are still many problems which are solved previously. In this study, initial flame behavior and flame propagation characteristic were investigated statistically in order to optimize combustion chamber shapes in the development stage of mass production S.I. engine. To the purpose, the authors applied the flame image capturing system to single cylinder optical engine. The captured flame images were effectively analyzed by using the image processing program which was developed by the authors and adopted new threshold algorithm instead of conventional histogram analysis. In addition, the cylinder pressure was also measured simultaneously to compare evaluated flame results with cylinder pressure data in terms of the combustion characteristics, combustion stability, and cycle-to-cycle combustion variability.