[en] Objective: To calculate the radioactivity disposition in endovascular irradiation for preventing restenosis after angioplasty and to give an empirical formula easy to use. Methods: 1) The process of intravascular brachytherapy with ⁹⁰Y, ¹⁸⁸Re or ³²P filled balloon was simulated with the computer program based on beta particle dose point kernel; and the absorbed dose distribution was calculated. 2) The curve of absorbed dose rate versus the outer radius of the balloon and the penetrating depth in the vessel wall were fitted to a function with non-linear least squares method and an empirical formula easy to use was built. Results: The absorbed doses distributing along the radial direction of the standard balloon showed that the peak value was inside the balloon, and the inflexion of the dose curve was at the endothelial surface of the vessel. The dose decreased sharply with the radial distance approximately in an exponential manner in the vessel wall. The relationship of absorbed dose to radiation time, initial radioactivity, tissue depth, balloon outer radius were all expressed with an empirical formula. Conclusions: The dose decreases sharply within the vessel wall, and that is very advantageous for focusing radiation energy on the vessel wall and preventing damaging distant normal tissue. Using the empirical formula, according to the required absorbed dose, the required radiation time can be easily calculated for clinical purpose