This paper describes the effect of traction oil on the power loss of spur gear. In this study, four traction oils and a turbine oil were used. The power loss was separated into gear friction loss and oil churning loss. Effects of gear torque, gear speed, and oil temperature on the friction loss were investigated. Besides, the flash temperature on the contact surface was measured using a dynamic thermocouple, and the lubrication condition was observed using electrical resistance. The relationship of the gear friction loss to the lubrication condition on spur gear is discussed.