As an integral part of the Chinese Society of Clinical Oncology Breast Cancer (CSCO BC) guideline, the radiation therapy (RT) section was issued in 2017 and updated annually. The panel members make risk-adapted stratified practical clinical recommendations of postoperative RT which reflect the latest global literature in the field and high-quality clinical trials from Chinese academic centers in a timely manner. Considering another function of encouraging physicians to participate in clinical trials, the footnotes of the postoperative RT section highlight the controversies that existed in decision making, the importance of modern RT techniques in decreasing toxicity, and the main evidence supporting treatment recommendations in each clinical scenario, including "grey zones". The postoperative RT section of the 2021 CSCO BC guideline can be summarized as follows: (I) prioritize the routine clinical practice of hypofractionated regime in postoperative RT, especially in breast-conserving patients receiving whole- breast irradiation without regional nodal irradiation (RNI). Apart from 3-week based moderated hypofractionated whole breast irradiation (WBI), the one week schedule is also included in the recommendation for selective node negative patients. (II) Refine risk-adapted decision-making of RT, including nodal irradiation in T1-2 N1 patients and omitting irradiation in low risk patients receiving breast conservative surgery; (III) recommend comprehensive criteria of postoperative RT technical details. The panel members hope that evidence-based recommendations continuously contribute to an improved standardization of BC postoperative radiotherapy and to synchronizing the clinical practice following the results of the latest high-quality clinical research.