Abstract: Air motion inside the cylinder of engine plays an important role in the processes of fuel spray and combustion, determine the economy, power output, exhaust emissions and combustion noise of the engine. The multidimensional simulation of the flow in cylinder can give the flow field information in detail and economically, show the effects of geometric and flow parameters to the combustion character, compare the combustion effect of different schemes, provide a effect research methods in engine development. It was simulated of air motion in a 4 valve S.I. engine in this thesis, the code used was a the KIVA3 code and the code was compiled by ALE method. Air motion inside the cylinder of 4 valve S.I. engine with two different length valve, and the affection of valve shape on the air flow and tumble flow strength in cylinder was analyzed. The calculated results indicates, the tumble flow was produced by the use of 4 valve S.I. engine, the tumble flow will be different with different shape valve, and the tumble flow strength with long valve will be stronger than that of short valve. These research results can provide some useful information for the design of S.I. engine.