Abstract Recently, functionally gradient materials (FGMs) based on polymer resins are widely developed. For example, FGM gears using polymer such as nylon, polybutylene terephthalate or polypropylene reinforced with other materials have been investigated. FGM gears are expected to improve strength, transmission efficiency, ignition loss and noise emission by concentrating reinforcing material on the tooth surface. In this study, we focus on thermal conductivity and heat resistance, which are essential for improving the strength of the gear tooth and consider the effect of position of reinforcing materials on FGMs by thermal-structural coupled finite element method analysis. As a result, we found that including reinforcing nanoparticle concatenations have a greater effect on thermal conduction than particle agglomeration and volume concentration. In addition, although nylon becomes high strain after thermal deformation, destruction will be considered to occur at the material interface between copper and nylon due to large difference of elastic modulus under thermal stress conditions.