Objective To compare the therapeutic effect of methimazole 15 mg/d and 30 mg/d for thyrotoxic Graves’ disease and to observe adverse reactions among newly diagnosed patients of this disease. MethodsTwo hundred and forty-five cases of Graves disease were randomly assigned to two therapy groups in a prospective study. Each patient in Group A(n=135) was given 15mg methimazole once daily in the morning while Group B(n=110) was given 10 mg methimazole three times a day. A follow-up study followed methimazole treatment at different initial dosages among all the participants. The remission rate and frequency of adverse effects were measured after 4, 8 and 12 weeks of treatment.Results (1)The rate of compliance with methimazole was 91.9% in Group A, which was significantly higher than 80.9% in Group B. (2) No remarkable difference of efficiency in normalizing FT4 and FT3 was observed between the two groups at the 4th, 8th and 12th week (62.1%, 83.6% and 89.7% in Group A compared with 65.9%, 84.4% and 88.3% in Group B). (3) The incidence of adverse reactions was 5.9% in Group A and 10.9% in Group B. (4)Thyroid enlargement of Ⅲ degree(OR=12.529) or four-time increase of serum FT4(OR=9.117)was a risk factor that influenced the treatment outcome of 15 mg methimazole once a day.Conclusions In the initial treatment ofg thyrotoxic Graves’ disease, 15 mg methimazole once a day is safe and effective for most patients, but for cases with thyroid enlargement of Ⅲ degree or significant elevation of thyroid hormone,10 mg methimazole three times a day is preferable.