This study has been performed to investigate on the spur gear with the spline key of heat treatment process analysis. The heat treatment process for gear has been widely used at the production of moving parts in order to improve its manufacture productivity. A driving spur gear is modeled with three dimensions for the heat treatment process. The variable conditions for the heat treatment analysis are the shape of gear tooth, the environmental heating temperature, heating time, cooling temperature and cooling. The analytical results of mechanical properties as micro structure and hardness are analyzed using the software. As the analysis result, the hardness value(HRC) becomes 20.2 of the inner part at gear body and of the surface part of gear tooth has shown the max. valve of HRC 47.5. The beinite structure forms up to 70%. This is because the cooling becomes slow at the center part than the end part. Therefore, it is estimated that the gear tooth have the high anti-wear property and the inner center part maintains the sufficient stiffness. These results can be utilized for the production of the driving gear by the heat treatment process as the fundamental information