The measurements of the temporal and cyclic fluctuation of the fuel vapor concentration were made close to the spark plug in the combustion chamber of a motored SI engine via laser Rayleigh scattering. The effects of several engine operating parameters on the concentration fluctuation were also examined. The experimental results showed that the concentration fluctuation was composed of the temporal concentration fluctuation in a specific cycle and the cyclic variation of the temporal mean concentration over many cycles, which could be separated and measured accurately with the present developed technique. It was noted that the concentration fluctuation increased during the intake stroke and reached a peak after which it decreased simply during the intake and compression strokes. It was also found that there was a strong correlation between the peak vapor concentration during the intake stroke and the vapor concentration at the end of the compression stroke in the combustion chamber of SI engine.