In order to investigate the characteristics of top ring groove deposit formation in gasoline and diesel engine, engine test and simulation test were performed.From component analysis of used oils sampled from actual running engines, oxidation and nitration for gasoline engine and soot content for diesel engine were selected as main parameters for evaluating oil degradation. In gasoline engine, deposit formation increases linearly with oxidation and nitration, and especially, oil oxidation is a dominant factor on the deposit formation rather than nitration. And, deposit formation increases gradually in low temperature ranges below 260°C even if oils are highly oxidized, but it increases rapidly if piston top ring groove temperature is above 260°C. In diesel engine, deposit formation is highly related to soot content in lubricating oils. Different from gasoline engine, soot contained in oil accelerates deposit formation even in low temperature range below 260°C, but the deposit formation is affected by both temperature and soot content in higher temperature range above 260°C. So, the deposit formation in IDI diesel engine, which produces larger amount of soot, is highly affected by soot as well as piston top ring groove temperature.