The aim of this study is to analyze the emission improvement in the oil-cooled engine by use of internal cylinder gas pressure measurement and 3D simulation of thermal flow and combustion.In the previous study, two test engines were designed to evaluate the benefits of the oil-cooled engine. One was an oil-cooled, and the other was a water-cooled engine. Both engines were single cylinder engines with SOHC valve-train systems. The hardware specifications of both engines were exactly the same except for their cooling systems in order to clarify how the difference in engine cooling system affects their cooling performance, warm-up performance and emission performance. Consequently, the oil-cooled engine showed a rise in temperature by about 80 K in the combustion chamber, resulting in a slight decrease in cooling performance, a 20% shorter warm-up time of the engine, while total hydrocarbons(THC) levels were lowered by 50% at maximum and nitrogen oxides(NOx) levels were almost equivalent to that of water-cooled engine in full-load operation.In this study, in-cylinder gas pressure measurement and 3D simulation of two test engines were conducted in full-load operation for clarifying the benefits of oil-cooling system with reproducing engine temperature and fuel evaporation. Thermal flow simulation was conducted to determine the temperature of combustion chamber surface and heat flux condition. Three-dimensional combustion simulation was conducted to evaluate how the difference in temperature of the combustion chamber affected the THC and NOx levels in emission. Both simulation conditions were based on previous experiment results.As the result of combustion pressure measurement, the difference in pressure values among the two engines were minimal indicating that the temperature of combustion gas is dominant in producing NOx rather than the temperature of combustion chamber. The result of steady state thermal flow simulation indicates that latent heat amount of fuel evaporation in the intake port and the cylinder of the oil-cooled engine were greater than that of the water-cooled engine. Finally, the result of three-dimensional combustion simulation based on thermal flow simulation indicates a higher combustion chamber temperature in the oil-cooled engine is more effective to reduce THC levels compared to water-cooled engine.