In order to control the mixture formation, a mixture injected 4-valve SI engine was developed with a small mixture chamber and mechanically driven mixture injection valve installed into the cylinder head. The mixture injection valve was located at the center of the combustion chamber. The mixture was injected from the final stage of the intake stroke to the beginning of the compression stroke. The mixture distribution and in-cylinder flow field inside the combustion chamber were measured by a pair of laser two-dimensional visualization techniques. A planar-laser-induced exciplex fluorescence technique was used to visualize the in-cylinder mixture formation by obtaining spectrally separated fluorescence images of liquid and vapor phase fuel distribution. Particle image velocimetry (PIV) was used to obtain flow field images. In the case of the mixture injected SI engine, the mixture injected into the swirl center was retained during the compression stroke. A high concentration of fuel vapor still remained and was distributed near the center of combustion chamber until the final stage of compression stroke. The impinged fuel on the cylinder wall was decreased as compared with that of a conventional port injection. Combustion characteristics and engine performance of the mixture injected SI engine were also measured under various engine operating conditions, and the relationship of combustion stability and mixture distribution was discussed. The mixture injected SI engine had a high combustion rate and high combustion stability under lean operating conditions in comparison with homogeneous combustion of a conventional port injection engine. The limit air/fuel ratio could reach over 40. It was clear that the optimization of mixture distribution was very effective to increasing combustion stability under lean operating conditions.