A LPG engine has the characteristics of lower NOx emission than the gasoline engine. In this study, in-depth consideration made this mechanism clear that the main cause of this NOx reduction was the decrease in local combustion temperature of LPG by high water content in its combustion products. In addition, increase of EGR was tested in order to reduce NOx emission more and to improve BSEC. As a result, combustion stability of the LPG engine is greater than that of the gasoline engine enough to improve these performances.