Most of the industrial machines use the belt transmission for the power transfer. These mechanisms often use the round belts with a few millimetres in diameter, which are made of the thermoplastic elastomers, especially the polyurethane. Their production process requires the bonding step, which is often performed by the butt welding, using the hot plate. To design the automatic welding machine in a proper way, the authors analyzed this process. One of the most important parameter which describes the hot plate welding is plasticized distance. Knowledge about its value is necessary to predict the length reduction of the belt during welding and will be used during the quality of the joint rating. To obtain value of this parameter the temperature distribution calculations are needed. The paper shows the results of the analytical calculations of the temperature distribution and their comparison with analyses performed by the FEM implements. Calculations were performed for the thermoplastic elastomer during the heating phase in the hot plate welding process.