The fuel mixture distribution has been measured at the spark timing and the combustion duration by the LIF technique in order to analyze the combustion fluctuation in a DI gasoline engine. It has been revealed that the combustion fluctuations under both the early and late injection conditions are dominated by the mixture concentration at the spark position and timing. On the other hand, for the best injection timing, the combustion fluctuations are strongly correlated to the unburned mixture quantity around the piston cavity during the latter combustion period. For improving the combustion fluctuations, it is effective to make a compact shaped mixture to reduce the over lean region.