An experimental study was made of the planar measurements of fuel vapor concentration in the combustion chamber of a spark ignition (SI)engine with laser-sheet-induced Rayleigh scattering. The conventional engine was modified to introduce YAG laser sheet light into the combustion chamber. The scattered light from the measuring plane was allowed to transmit an optical glass window installed on the extended piston and captured with a CCD camera fitted with a gated doublemicrochannel-plate image intensifier by lens coupling. The time histories of fuel vapor concentration distribution and the effect of injection methods on them were investigated thoroughly. It was found that the fuel vapor concentration was highly heterogeneous during the intake stroke, and the inhomogeneity decreased in the compression stroke. However, even at the end of the compression stroke, small lumps of unhomogeneous mixture still existed randomly inside the engine combustion chamber.