This paper presents the effect of fuel atomization at an intake manifold on the combustion characteristics of a spark-ignition engine. Four sets of fuel-supply devices were tested which have much difference in atomization characteristics in terms of the mean droplet size ranging from wall film to 7 um S.M.D. Over the whole operatable range of mixture strength, the fuel atomization did not give any effect on BMEP, BSFC and volumetric efficiency, but had an effect on the engine stability at lean operation limit under most operating conditions. This atomization effect was shown to be systematically characterized by mapping on the ignition timing and A/F plane. The appearance of partial-burn limit on the map was successfully verified by the combustion analysis using a correlation procedure on cycle by cycle basis. At higher engine speed, the atomization effect was suppressed, which is considered to be caused by the contribution of the increased intake air velocity to the improvement of secondary atomization. At lower load, the atomization effect was promoted, which must be due to the suppression of the secondary atomization with decreased intake air velocity. However, for lower load and M.B.T. condition, the engine stability parameter was not useful for characterizing the effect of fuel atomization.THE FUEL BEHAVIOR in an intake manifold and its effect have been studied since old days. When a carburetor was the only device for supplying fuel, the atomization effect on the engine performance was examined and an effect of the extention of lean operation limit was reported(1). Due to the exhaust emission regulations, the main device has been replaced to an electronically controlled fuel injection. These days, much more severe regulations, which are so-called LEV and ULEV, are going to be introduced, and the importance of the fuel preparation has increased. Thus, several studies were carried out to reveal the effect of fuel preparation behavior for the engine combustion(2),(3). Recently, a specially designed engine for examining a transient condition was invented, and a visualization study around the intake system and a study for searching the HC origin were reported(4),(5). Thus, the fuel preparation study or the mixture formation study in spark-ignition engines have been increasingly important.The authors carried out a study on the relationship between the fuel-supply manner and the combustion characteristics, and showed that it systematically, determines the lean-operation limit(2). However, there were several limitations of operating conditions and the variations of fuel-supply devices.Thus, this study was conducted to see the more general effect of fuel atomization on the combustion characteristics in a spark-ignition engine. For this purpose, four sets of fuel supply systems were prepared in terms of the fuel atomization and the mixture formation control. Experiments were carried out under steady conditions and results of basic aspect of the effect of fuel preparation on the engine combustion characteristics are reported here.