Abstract: Objective To study the factors influencing therapeutic effect of ¹³¹Ⅰ in treating 783 cases of Graves disease. Methods The values of various indexes were quantized on influencing factors such as age, sex, course of disease, mass of thyroid gland, the absorbed dose of ¹³¹Ⅰ per gram of thryoid gland, the given dose of ¹³¹Ⅰ, thyroid 24 h ¹³¹Ⅰ uptaking percentage, thyroid hormone and thyroid autoantibodys. The assessment of the therapeutic effect was made according to complete remission (including hypothyroidism) and partial remission after ¹³¹Ⅰ therapy. CMH χ², Wilcoxon signed-rank test and Logistic regression were used to analyze the variable parameters before the treatment. Results The therapeutic effect of ¹³¹Ⅰ is significant in the follow-up period (CMHχ²=69.21, P<0.01). The mass of thyroid, rashness, bulimia, etc, were relieved significantly(all the values of P<0.05) after ¹³¹Ⅰ therapy for 12 months. The therapeutic effect was related to such factors as age thyroid mass, the absorbed dose of ¹³¹Ⅰ per gram of thryoid gland, thyroid 24 h ¹³¹Ⅰ uptaking percentage, etc (all the values of P<0.05). Conclusion In those older patients and those whose thyroid 24 h ¹³¹Ⅰ uptaking percentage is higher, thyroid mass is bigger, thyroid gland is with noduses, the given dose of ¹³¹Ⅰ should be increased. Conversely decreased.