Objective：To investigate the efficacy of 131I therapy in patients with Graves’ disease（GD） and its influencing factors. Methods：A retrospective analysis was performed for 497 GD patients treated with single 131I therapy at the outpatient service of the Nuclear Medicine Department of Nanjing Drum Tower Hospital from 2013 to 2016. These patients were followed up at 3，6，12，and 36 months after therapy. The general data of patients，such as age，sex，the duration of disease，symptoms，and signs，as well as the findings on thyroid color Doppler ultrasound，thyroid iodine uptake rate，thyroid function，thyroid autoimmune antibodies，and the dose of 131I，were collected to assess the treatment outcome of 131I therapy. The independent-samples t test，chi-square test，and logistic regression were used to analyze the efficacy of 131I therapy in GD patients and its influencing factors. Results：The response rates based on the follow-up performed at 3，6，12，and 36 months after therapy were 75.6%，79.4%，93.9%，and 94.7%，respectively. Compared with the hyperthyroidism or normal group，the hyperthyroidism group had significantly longer duration of disease and duration of use of antithyroid drugs before radioactive iodine（RAI） therapy and significantly higher thyroid weight，2-h iodine uptake rate，thyroid stimulating hormone receptor antibody titer，and 131I dose per gram of thyroid（P<0.05）. The treatment response of GD patients with abnormal liver function was not inferior to that of those with normal liver function（P=0.001）. A higher response rate was seen in the patients taking propranolol before RAI therapy（P=0.000）. Compared with those with poorer response to treatment，the GD patients with better response to treatment had a significantly shorter duration of hyperthyroidism（OR=0.939，95%CI＝0.895 to 0.986，P=0.011） and a significantly lower thyroid weight（OR=0.961，95%CI＝0.937 to 0.986，P=0.002）. Conclusion：The effi－cacy of 131I in the treatment of GD tends to increase over the course of treatment. Thyroid weight and the duration of disease are main influencing factors for the efficacy of 131I therapy for GD；Higher thyroid weight and longer duration of hyperthyroidism are associated with poorer treatment response.