Abstract: Objective Effect of all kinds of determined parameter (example for geometry seat, self-shield, container and thickness of wall of container, etc) on the apparent activity of ¹²⁵I sealed seed source was briefly described in this paper, the optimal condition of measurement was established. Methods Iodine-125 sealed seed source is inserted erectly in the lml peaked bottom tube, and is determined with CRC-15 radioactive calibrator. Results The condition of measurement can affect the apparent activity value of ¹²⁵I sealed seed source. Conclusions Theapparent activity measurement of ¹²⁵I sealed seed source was exact under the optimal condition.