The thermoelectric materials can convert electricity and heat directly. The practical use is expected at a point to reuse unused heat. In the thermoelectric module made from many thermoelectric materials, the performance evaluation in large temperature difference is very important. The output power, the heat flow, the conversion efficiency, and so on, can be evaluated by the measurement in large temperature difference. The heat flow is evaluated by the temperature gradient method or the flow calorimeter. ADVANCE RIKO Inc. manufactured evaluation instruments (Mini-PEM, PEM-2, F-PEM). The conversion efficiency is calculated from the output power and the heat flow. In this article, we introduce the theory, the description of equipment, the evaluation of the heat flow, the measurement example, and the present condition.