This paper proposes a simple calorimetric power loss measurement system for highly efficient converters using a single chamber and a Peltier device with ambient temperature tracking control. In the proposed system, a mechanical heat pump and a heat exchanger which increase the cost of the calorimeter are not required. The power loss of a converter under test is measured by the heat absorption of a single Peltier device. Also, the ambient temperature tracking controller minimizes the heat leak which decreases the measurement accuracy without using a complex double chamber structure. In this paper, the principle of the proposed calorimetric measurement method and the proposed system are described. As the experiment result, a heat measurement accuracy of less than 2% can be obtained at 15 W condition. The measurement time of the proposed method is less than 1500 seconds, which is shorter than the conventional closed type double-cased method.