Abstract: As a functionally unique subset of T cells, CD₄⁺CD₂₅⁺ regulatory T cells (Treg) can suppress effective immune responses.These T cells play a critical role in immunologic self-tolerance, anti-tumor immune re-sponses, transplantation and so on.Foxp3, CD127, GITR and other molecules affect the regulatory role of Treg.Increasing evidence supports the existence of elevated numbers of regulatory T cells in solid tumors and hematologic malignancies.In patients with breast, ovarian, lung or hepatic cancers, Treg levels are in-creased not only in the tumor microenvironment but also in the peripheral blood.Poor prognosis and de-creased survival rates are closely correlated with higher Treg cell frequencies, and in patients with hematolog-ic malignancies the number of Treg is increased as the stage advances.However, little is known about the mechanisms leading to this phenomenon.Depletion of Treg or blockade of their immune inhibitory role can en-hance anti-tumor effects.Chemotherapy influences the number of Treg in the peripheral blood of cancer pa-tients.Chemotherapeutics may inhibit vascularization in the tumor microenvironment and promote apoptosis of Treg to suppress the tumor growth.Patients treated with Cyclophosphamide, Docetaxel or Fludarabine ex-hibit significant tumor regression and prolonged survival, but there are also some studies reporting that the number of Treg in the peripheral blood is increased after treatment.The exact effect of chemotherapy on Treg is uncertain and needs to be further investigated.Depleting or reversing the regulatory effect of Treg is key in tumor immunotherapy.