Abstract: Objective:To investigate the effects of CD 4+CD25+ regulatory T cells (Tregs) on the epithelial-mesenchymal transition (EMT), migratory and invasive capacities, and proportion of ALDH1+ cancer stem cell (CSC)-like cells in breast cancer cells. Methods:CD4+ CD25+ Tregs were isolated from the peripheral blood of breast cancer patients through immunomagnetic sand method. In the co- culture groups, breast cancer cell lines BT 474 and MCF- 7 were co-cultured with CD4+ CD25 + Tregs. In the control groups, BT 474 and MCF- 7 cells were cultured alone. EMT-related markers of breast cancer cells were then assayed in the control and co culture groups. The migrato-ry and invasive capacities of breast cancer cells were also detected in both groups. Furthermore, the changes in ALDH 1+ CSC-like cells, mammosphere-forming ability, and self-renewal capacity of BT474 cells were assayed. Results:After co-culture with CD 4+ CD25+ Tregs, the expression of mesenchymal biomarkers increased in BT474 and MCF- 7 cells, whereas the expression of the epithelial biomarker E-cadherin decreased in MCF-7 cells. Additionally, the migratory and invasive capacities of BT 474 and MCF- 7 cells increased after co-cul -ture. Overall, the subpopulation of ALDH 1+ CSC- like cells, the mammosphere- forming ability, and the self- renewal capacity of BT474 cells improved in the co-culture group compared with the control group. Conclusion: The EMT process, migratory and invasive capaci -ties, and CSC-like properties of breast cancer cells can be enhanced by CD 4+ CD25 + Tregs.