Naturally occurred CD4(+)CD25(+) regulatory T cells derived from thymus. It plays an important role in self-tolerance and allograft-tolerance through cell-contact dependent mechanism. This review described the advances of study on the probable regulatory factors of the naturally occurring regulatory T cells, such as Foxp3, IL-2, TGF-beta(1), dendritic cells and CTLA-4. As a marker of Treg, the expression of Foxp3 could be used to identify regulatory T cells. The combination of interferon 2 and IL-2Ralpha would activate Treg and promote its proliferation through the phosphorylation of STAT5. TGF-beta(1) on the cell surface may influence the function of Treg, while the secretion type of TGF-beta may promote the proliferation of Treg. Dendritic cells can positively or negatively regulate Treg, which depends on the signal transduction pathway. CTLA-4 expressed on the surface of Treg might bind to the B7 molecule on the DC, effective cell or Treg itself directly or indirectly regulate Treg.