To improve performance of motorcycle 4 cycle 250 cc engine, we examined the following. (1) We improved crank balance so that friction loss of crank shaft was reduced. (2) We reduced air intake resistance and improved exhaust system so that volumetric efficiency was increased. (3) We reduced weight of valve system parts so that maximum RPM was increased. As these improvement, we have achieved 33.1 kw/15000 RPM and torque increase in low and middle RPM range for motorcycle 250 cc engine on 1989 year model.