Purpose: To evaluate the outcome of ductal carcinoma in situ (DCIS) patients who underwent surgery followed by radiation therapy (RT). Materials and Methods: We retrospectively reviewed 106 DCIS patients who underwent surgery followed by postoperative RT between 1994 and 2006. Ninety-four patients underwent breast-conserving surgery, and mastectomy was performed in 12 patients due to extensive DCIS. Postoperative RT was delivered to whole breast with 50.4 Gy/28 fx. Tumor bed boost was offered to 7 patients (6.6%). Patients with hormonal receptor-positive tumors were treated with hormonal therapy. Results: The median follow-up duration was 83.4 months (range, 33.4 to 191.5 months) and the median age was 47.8 years. Ten patients (9.4%) had resection margin <1 mm and high-grade and estrogen receptor-negative tumors were observed in 39 (36.8%) and 20 (18.9%) patients, respectively. The 7-year ipsilateral breast tumor recurrence (IBTR)-free survival rate was 95.3%. Resection margin (<1 or ${geq}1$ mm) was the significant prognostic factor for IBTR in univariate and multivariate analyses (p < 0.001 and p = 0.016, respectively). Conclusion: Postoperative RT for DCIS can achieve favorable treatment outcome. Resection margin was the important prognostic factor for IBTR in the DCIS patients who underwent postoperative RT.