To evaluate the effectiveness of I-131 in ablation of residual thyroid tissue, we analyzed 350 patients with thyroid cancer who were treated with various doses of I-131 after surgery for thyroid cancer. Two hundred fifty five patients were treated with l.lGBq(30mCi) of I-131 for ablation of remnant thyroid and one hundred seventeen patients received more than 2.8GBq(75mCi) of I-131. We determined the effectiveness of ablation by following I-131 whole body scan. Absent visible uptake or minimal uptake in thyroid tissue were considered as successful ablation. Of 255 patients who received doses of 30mCi I-131 therapy, 131 patients(51%) showed successful ablation of residual thyroid tissue with 2.6±1.7 times of I-131 therapy. Of 117 patients who received doses of the more than 75mCi I-131, 84 patients(72%) had successful remnant thyroid ablation with 1.6±1.1 times of I-131 therapy. According to the extent of surgery, successful ablation rates were 78%, 62%, 54%, 33% in patients who underwent total thyroidectomy, subtotal thyroidectomy, lobectomy and isthmectomy, lobectomy or tumorectomy, respectively. This study showed that ablation of remnant thyroid after surgery with 30mCi I-131 was successful only in 50%. Therefore, in cases of patients with high risk for recurrence, we recommend high dose I-131 for ablation of remnant after total thyroidectomy.