An experimental study was carried out on the two-dimensional measurements of the cyclic variation of mixture concentration distribution in the combustion chamber of a spark ignition engine with the PLIF (planar-laser-induced fluorescence) technique. Nitrogen dioxide was selected as the fluorescence tracer to simulate the fuel. A commercial engine was modified to enable visuable diagnosis. A theoretical analysis was carried out to define the cyclic variation of mixture concentration distribution. The results show that the local cyclic variation of heterogeneity of mixture concentration distribution is marked over the measurement plane, which is assumed to be caused by the randomly distributed mixture lumps in a specific cycle whose distributions also change greatly from one cycle to the next. The time histories of cyclic variation and the effects of engine operating parameters including the fuel injection timing and the engine speed were elucidated.