The automatic transmission has a mechanism that transmits the power of engine to the driving system of the vehicle. While power is transmitted, heat is generated by gear friction, bearing friction, clutch friction, and oil churning, which are internal parts of the automatic transmission. If the temperature rises inside the transmission, it may adversely affect performance and durability, as well as affect malfunctions such as electric sensors. So, it is necessary to cool the heat generated inside the transmission. In this paper, we developed a thermal prediction model in vehicle condition. The model was composed of the vehicle model, controller, powertrain, and cooling circuit of the vehicle. We measured the temperature at each position of the transmission according to the driving conditions and thermal model verified by comparing experiment results.