Abstract: A simple-structure plutonium calorimeter was developed and the size of sample cell is φ185 mm and high 200 mm. The performance tests show that the sensitivity of calorimeter is about 202 mV · W-1 and the linear scope of the measuring thermal power is from 4 mW to 9 W. The thermal power of plutonium was measured by the calorimeter. And it was compared with that from chemical analysis and weighting method. The results show that a good precision could be expected to measure thermal power of plutonium sample by this apparatus. The precision of the calorimeter is better than 0. 5%, when the thermal power of sample is not lower than 200 mW.