Abstract: Regulatory T cells (Treg), a group of negative regulatory cells, have four subsets: CD4⁺ Treg, CD8⁺ Treg, NKT Treg and DN Treg cells. They play an essential role in the inhibitive immune-regulation and might be the key factors of neoplasms immune escape. These mechanisms include inhibiting the effector cell function by inhibitory cytokines, killing effector cells by granzyme and perforin, competition and inhibiting IL-2, and affecting Treg differentiation and proliferation by regulating the function of CTLA-4, etc. Tumor immunotherapies targeting Treg and related immunosuppressive factors, such as remove Treg or controling the numbers and functions, enhances the immune response against tumors, which might  offer a new method of tumor immunotherapy.