This study established the function of the retained body activity and the external dose rate of patients treated by 131I. The retained body activities of patients can be estimated by the external dose rate based on this function. The external dose rate and retained body activity of sixty-eight patients were measured. The external dose rate was measured at 0.5 h, 4 h, 8 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h and 192 h after the administration with distances of 0.3 m, 0.6 m, 1 m, and 2 m, respectively, with the urine received at the same time for the calculation of the retained body activity. The function of them is Y=22.8X+3.3; Y stands for the retained body activity (MBq), X stands for the external dose rate of patient at 1m (μSv/h). This study is beneficial for radiation safety decision-making.Cited