The objective of the present study is to clarify the effect of early-closing of an intake-valve on the engine performance in a spark-ignition engine. For the first step of the study, under natural aspirating condition, four sets of expansion ratio pistons of 11, 16, 20 and 22 were prepared for two kinds of camshaft, one is, original and the other is geometrically half early-closing.. The obtained performance data and the indicator pressure records were analyzed. It was shown that up to 7 % of improvement in the thermal efficiency can be realized over a wide range of operating condition. This is considered to be mainly caused by the effect of the increased-expansion cycle. The increased-expansion effect can be estimated to be about 1.4 which almost corresponds to the geometrical inlet-valve closure timing. However, this value is not consistent with the measured volumetric efficiency which was almost half the value associated with original timing. This inconsistency is discussed, and the increase of the residual gas fraction is suggested to be the main reason for it. This suggestion was verified qualitatively by a combustion analysis using a two-zone heat release model.