In this study, three dimensional heat conduction analyses of piston in gasoline engine were performed by boundary element method. In order to obtain the boundary conditions for the gas temperature in cylinder and heat transfer coefficients on piston wall, diagnostic engine performance simulations were performed. In order to verify the appropriateness of boundary element method in the 3 dimensional heat conduction analysis of piston temperatures, surface temperatures of piston wall were measured. In this experiment L-link mechanism is used for leading the thermocouples from the piston to the outside of engine. Since most of the analytical results with various operating conditions agree reasonalby well with the experimental ones, appropriateness of boundary element method in the 3-dimensional heat conduction analysis was in some degree verified. However since there also exist some local points where the analytical results do not agree well with the experimental ones, further studies on the more accurate verification of boundary element method are required. For the purpose of them, study on the more accurate establishment of heat transfer coefficients on piston wall is required.