Background The role of adjuvant radiotherapy (RT) for patients with stage Ⅲ thymoma after complete resection is not definite.Some authors have advocated postoperative RT after complete tumor resection, but some others suggested observation.In this study, we retrospectively evaluated the effect of postoperative RT on survival as well as tumor control in patients with Masaoka stage Ⅲ thymoma.Methods Between June 1982 and December 2010, 65 patients who underwent complete resection of stage Ⅲ thymoma entered the study.Fifty-three patients had adjuvant RT after surgery (S+R) and 12 had surgery only (S alone).Of patients who had adjuvant RT, 28 had three-dimensional conformal RT (3D-CRT)/intensity modulated RT (IMRT) and 25 had conventional RT.A median prescribed dose of 56 Gy (range, 28-60 Gy) was given.Results The median follow-up time was 50 months (range, 5-360 months).Five-and 10-year overall survival (OS) rates were 91.7% and 71.6%, respectively, for S+R and 81.5% and 65.2% for S alone (P=0.5).In the subgroup analysis, patients with 3D-CRT/lMRT showed a trend of improved 5-year OS rate compared with conventional RT (100% vs.86.9%, P =0.12).Compared with S alone, the 5-year OS rate was significantly improved (100% vs.81.5%, P=0.049).Relapses occurred in 15 patients (23.1%).There was a trend of lower crude local recurrence rates for S+R (3.8%) compared with S alone (16.7%) (P=0.09), whereas the crude regional recurrence were similar (P=0.9).No clear dose-response relationship was found according to prescribed doses.Conclusions Adjuvant 3D-CRT/IMRT showed potential advantages in improving survival and reducing relapse in patients with stage Ⅲ thymoma after complete resection, whereas adjuvant RT did not significantly improve survival or reduce recurrence for the cohort as a whole.Doses of ≤50 Gy may be effective and could be prescribed for adjuvant RT.To confirm the role of adjuvant 3D-CRT/IMRT in patients who undergo a complete resection of thymoma, a multicenter randomized study should be performed.