The strength of the plastics varies greatly with temperature. Therefore, reduction of the meshing teeth temperature will increase the lifetime of the plastic gears. The reason to use plastic crossed helical gear in combination with metal gear is seems to reduce the increase in meshing teeth temperature due to the high thermal conductivity of the metal gear. We performed the endurance tests for the plastic crossed helical gear in combination with metal gear. The experimental results were as follows; (1)The failure mode of plastic crossed helical gear in combination with metal gear under no-lubrication condition is tooth breakage. (2)The lifetime of the plastic crossed helical gear in combination with metal gear is longer than plastic crossed helical gear. (3)There is a possibility that the lifetime of the plastic crossed helical gear in combination with metal gear under no-lubrication condition can be evaluated by the stress ratio.