Objective Despite the long experience in using radioiodine for treatment of the Graves' disease (GD), controversy still remains regarding the method to determine the optimal dose to achieve quick control of hyperthyroidism. The aim of this study was to examine the clinical outcome of 131I therapy with two different activities for patients. Methods We carried out a retrospective study to compare the therapeutic efficacy of two different 131I activities between <3.7 MBq/mL and >4.44 MBq/mL in 147 Graves' disease patients after a one-year follow-up period. Diagnosis of Graves’ disease was confirmed in patient’s hyperthyroidism symptoms and biochemical thyroid function tests, and met the following additional criteria, such as diffusely enlarged thyroid gland without palpable nodules and an elevated uptake of radioiodine by the gland. Results Overall 48.0% or 94.8% of the patients had successful treatment outcome in the two groups that were subjected to lower and higher radioactivity, respectively. The cure rate including both euthyridism and hypothyroidism incidence rate (confirmed by serum TSH and free T3/free T4)). There was significant difference of outcome in the cure rate and the reduced rate of thyroid volume between the lower and higher activities of radioiodine groups. Conclusion Radioiodine higher than 4.44 MBq/mL was found to be more effective than dosage less than 3.7 MBq/mL in managing patients with Graves' disease. The higher activity (4.44-5.18 MBq /mL) should be routinely used in patients.