The polonium-210 activity in raw material targets which was used to make the start neutron source rods of Qinshan Nuclear Power Station was determined in calorimetry. The calorimeter used is a twin-cup isothermal one. It works at room temperature and does not need any device of constant temperature. The instruments used for calorimetry are simple and have good properties. The background is from -1 µ V to +2 µ V when the room temperature changs between 16℃ and 28℃. The data gotten in calorimetry are in correspondence within ±3.4% with those from liquid scintillation method after the targets were dissolved, additionally in correspondence within ±1.7% with the data given by Amersham Laboratories for ²⁴¹Am determination.