Abstract: Objective: To understand the sources of radioactive waste gas discharge from the nuclide I-131 treatment site and find out the main pollution way to the environment. Method:A I-131 treatment site of a hospital was selected. Aerosol samples of the air inside fume hood of the high-activity room and thyroid cancer treatment ward, organized emission such as the ventilating system emission of the fume hood and the treatment ward, unorganized emission such as air leaked from windows of the wards and cover plate of the decay tanks were collected, and the activity concentration of I-131 was analyzed. Results:The results showed that the activity concentration of I-131 from high to low is inside the fume hood of the high-activity room, ventilation system emission of the fume hood, inside the thyroid cancer treatment ward, ventilation system emission of the ward, air leaked from the cover plate of the decay tank and from windows of the wards. Conclusion:The fume hood inside the high-activity room, as a storage and subpackage device of the radiopharmaceutical, is the main discharge source of a I-131 treatment site and therefore more attention should be paid to its ventilation design. Effluent monitoring also should be enhanced to make sure the filter is under normal operation. Furthermore, contribution of fugitive emission source terms such as air leakage from the window of the treatment wards and the cover gap of the decay tanks to the activity concentration of I-131 in the air of the region close to the I-131 treatment site. Considering that the organized discharge facilities usually located on the top of the building and three meters taller than the tallest building within fifty meters scope around the treatment site, environment impact assessment of the area close to the I-131 treatment site should be focused on the unorganized emission sources.