Objective To compare the clinical efficacy between breast-conserving surgery (BCS) plus radiotherapy (RT) and modified mastectomy in patients with stage Ⅰ breast cancer in clinical setting.Methods Clinical data of 6 137 patients diagnosed with pT1-2N0 breast cancer from 1999 to 2014 were retrospectively reviewed.Among them,1 296 patients received BCS plus RT (BCS group) and 4 841 cases underwent modified mastectomy alone (modified mastectomy group).Kaplan-Meier analysis was used for survival analysis.Log-rank test,single factor analysis and Cox's proportional hazards regression model were performed.The results were further confirmed with the propensity score-matching (PSM) method.Results Within a median follow-up period of 55.2 months (range,1-222 months),the 5-year locoregional recurrence-free survival (LRFS),distant metastasis-free survival (DMFS),disease-free survival (DFS) and overall survival (OS) were 96.3％,93.7％,91.9％ and 96.9％,respectively.In the BCS plus RT group,the 5-year DMFS (96.9％ vs.92.9％,P＜0.001),DFS (94.9％ vs.91.2％,P=0.005) and OS (99.1％ vs.96.4％,P=0.001) were significantly higher than those in the mastectomy group.Multivariate analysis revealed that postoperative RT was an influencing factor of DMFS (P=0.003,HR=0.621;95％CI:0.455-0.849) and OS (P=0.036;HR=0.623;95％CI:0.401-0.969).For 1 252 pairs of patients matched by PSM,the 5-year OS (99.1％ vs.96.1％,P=0.001),DMFS (97.0％ vs.92.2％,P＜0.001) and DFS (95.3％ vs.90.2％,P=0.001) in the BCS plus RT group were significantly higher compared with those in the mastectomy group.Conclusion The long-term clinical prognosis of patients with stage Ⅰ breast cancer in the BCS plus RT group is better than that in the mastectomy group.BCS plus RT should be recommended for patients with stage Ⅰ breast cancer.