The strength of plastic greatly varies with temperature
that is, the strength increases as the temperature decreases. Therefore, the lifetime of a plastic gear can be increased by reducing the meshing-Teeth temperature of the gear. The meshing-Teeth temperature can be reduced by changing the material of the mating gear from plastic to metal considering that a metal gear will have high thermal conductivity. In this study, we investigated the load-carrying characteristics of an unlubricated crossed helical gear consisting of a plastic gear meshed with a metal gear. We performed gear lifetime experiments and meshing-Teeth temperature survey experiments. The results showed that the failure mode of the plastic crossed helical gear was caused by the breakage of the tooth and the rim. Then, we confirmed that the stress ratio could be an index for the lifetime evaluation of the plastic crossed helical gear that could fail by tooth breakage. Additionally, by testing gear pairs with several dimensions, we confirmed that the mean flash temperature could be an index for the meshing-Teeth temperature evaluation of the plastic crossed helical gear. Furthermore, we proposed a lifetime estimation method for the plastic crossed helical gear that could fail by tooth breakage and verified the validity of the proposed method based on the experimental results.