Abstract: Objective: To investigate the effect of chemotherapy on Treg (CD₄⁺CD₂₅⁺ regulatory T cell) in the peripheral blood of patients with breast cancer who received radical mastectomy or modified radical mastectomy. Methods: A total of 52 samples of the peripheral blood of post-operated patients with breast cancer were collected on the 1st day before chemotherapy and the 10th day after chemotherapy. The proportion of Treg and CD₃⁺, CD₄⁺, and CD₈⁺ T cells in the to-tal amount of T cells was evaluated by flow cytometry. The cytokine such as IL-4, IL-10, TGF-β 1 and IFN-γwere mea-sured by ELLSA. Results: The proportion of Treg in the peripheral blood was smaller before chemotherapy than after chemotherapy (P<0.05). The changes in Treg was not correlated with lymph node status, the number of positive lymph nodes and menopause status(P>0.05). No statistical significance was found in the percentage of CD₃⁺ and CD₄⁺ T cells in the peripheral blood between the samples of pre-and post-chemotherapy (P>0.05). The proportion of CD₈⁺ T cells in Tlymphocytes increased from (24.876± 6.631)% before chemotherapy to (28.129± 10.900)% after chemotherapy(P<0.05), but CD₄⁺/CD₈⁺ decreased from (1.680± 0.704)% to (1.506± 0.691)%(P<0.05). The levels of IL-4, IL-10 and TGF-β 1 after chemotherapy were lower than those before chemotherapy (P<0.05), but the level of IFN-γwas higher after chemotherapy (P<0.05). However, the changes in Treg were not correlated with the changes in the level of cytokines such as IL-4, IL-10, TGF-β 1 and IFN-γ. Conclusion: Chemotherapy may inhibit tumor through reducing Treg in the peripheral blood of patients with breast cancer.