In practical lean burn engines used to date, the use of a stratified air-fuel configuration, with a comparatively rich mixture in the vicinity of the spark plugs, has resulted in the stable combustion of an overall lean mixture. However, because a comparatively rich mixture is burned during the first half of combustion, NOx emissions are not reduced sufficiently.This research focused on a form of lean burn with homogeneous premixture that would be able to balance low NOx emissions with combustion controllability. It is widely known that homogeneous lean premixed gas has poor flame propagation characteristics. To determine the dominant cause of this, this study investigated the combustion properties of a single-cylinder engine while changing the compression ratio and intake temperature. As a result, the primary cause of combustion fluctuation, the abnormal cycle has a low TDC temperature compared to that of other cycles. It was clear that cycles with an unburned gas temperature in TDC below 950 K had particularly poor heat production during the expansion stroke.The effect of the temperature of the unburned mixture on the laminar burning velocity using a chemical reaction calculation indicated that the laminar burning velocity increased rapidly when the temperature of the unburned mixture exceeded approximately 1,000K. An investigation of the reason for this phenomenon by conducting sensitivity analyses for the chemical reactions contributing to the laminar burning velocity showed that when the temperature of the unburned mixture reached 1,000K or more, the decomposition of H2O2 produced OH radicals, which promoted combustion. The authors therefore made the modifications to a mass-production engine in order to help ensure that TDC in-cylinder temperature exceeded approximately 1,000K during lean burn. As a result, stable combustion in A/F30, the balance of -low NOx emissions with high efficiency could be achieved.