Objective To explore the difference between the mass of the thyroid determined by Computed Tomography (CT) and Emission Computed Tomography (ECT) scan in Graves’ disease (GD) patients with equivalent baseline clinical characteristics before they get their first 131I treatment, and to compare the short-term treatment effects between the patients whose thyroid gland masses were estimated by the two different ways. Methods Graves’ disease patients who were treated with 131I for the first time between Jan.2007 and Dec.2008 in PLA 307 hospital were included, whose age, length of disease course, thyroid size, thyroid iodine uptake rate, 131I effective half-life and planed 131I dosage per kilogram thyroid mass all met the inclusion criteria of this trial listed in the article below. Then patients were evaluated with CT (test group) and ECT (control group) to calculate their thyroid mass value. The ultrasound test was performed to estimate their thyroid weight which was used as a reference parameter to be compared with the CT and ECT results. The same formula was used to calculate the 131I treatment dosage and the same drug delivery method in both test and control group. 3 months later, and the patients’ therapeutic effects (invalid, improved, cured and hypothyroid) were evaluated in accordance with their clinical symptoms and thyroid function test results, and made comparisons between test and control group patients. Results In total, 112 patients were included in this study (60 in test group, 52 in control group, none statistically significant difference in baseline between two groups). CT determined thyroid mass value was higher than ultrasound result (average 70.28±18.39 vs 50.21±13.82，P＜0.05; linearly dependent, r=0.903，P＜0.05 ), ECT determined mass value was also higher than ultrasound result (average 79.26±24.57g vs 42.22±13.02g，P＜0.05; linearly dependent, r=0.880，P＜0.05 ).The average mass in ECT group was more heavier than that of in CT group (mean deviation from ultrasound result, 37.03±14.50g vs 20.06±8.31g，P＜0.05 ). After 3 months follow-up, there was no difference in invalid patient rate between the two groups (3/60 vs 1/52，P＞0.05), and there was more improved (51.7% vs 25.0%) but less cured (36.6% vs 53.9%) and hypothyroid (6.7% vs 19.2%) patients in test group (P＞0.05). Conclusion The mass of the thyroid gland determined by CT is heavier than that determined by ultrasound but lighter than the ECT test result. Compared to ECT evaluation, using CT scan to evaluate thyroid weight, more patients with GD treated by 131I get improved but less be cured and converted to hypothyroidism, though the valid rate (improved, cured and hypothyroid in all) is similar in two groups