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In order to obtain more reduction of two-stroke motorcycle engine noise than usual, it becomes necessary to make improvements within the combustion process itself. This study was carried out for two objectives. One is the investigation of the relationship between combustion and noise, and the other one is the development of noise reduction techniques. As the result, it was discovered that there was a significant correlation between engine noise and (dP/dθ)max, called the maximum rate of cylinder pressure rise. Therefore, the reduction of the (dP/dθ) max was recognized to be effective for engine noise reduction. The optimized alteration of combustion chamber shape is the most effective noise reduction technique, because it is able to reduce (dP/dθ) max without any sacrifice of engine power. In fact, the level of noise reduction can be predicted by one of the parameters obtained from the combustion chamber shape.