As a new identified help T cell lineage different from Th1 and Th2 cells, Th17 cell has been found played important roles in the pathogenesis of autoimmunity and inflammatory disease. To further identify their roles, the differentiation and regulation of Th17 cells has been widely explored recently. Now it has been confirmed that TGF-beta, combined with IL-6 or IL-21, play critical roles in the differentiation of Th17 cells. While IL-23 mainly contribute in promoting the secretion of IL-17 and maintaining the function of Th17 cells. Corresponding with the Th1,Th2, and Treg cells, which has special transcription factors T-bet、GATA3、Foxp3 respectively, now it has been confirmed that ROR-γt(retinoid-related orphan receptors-γt) is the special transcription factor which specially regulate the differentiation of Th17 cells. Th17 cells function through their secreted pro-inflammatory cytokines, including IL-17A, IL-17F, IL-21, IL-22, IL-6, TNF-α. Among them IL-21，which act as a autocrine cytokine of Th17 cells, play critical roles in promoting the differentiation of Th17 cells while inhibiting the differentiation and function of Th1 and Treg cells. On the other hand, IL-2, which is obligatory for the growth of Th1,Th2,Treg and CD8⁺T cells, now has been found negatively regulate the differentiation of Th17 cells. In all, differentiation of Th17 and Treg,Th1 cells are exactly regulated in vivo, in which TGF-beta played critical roles. As both Th1 and Th17 cells participate in the pathogenesis of autoimmunity and inflammatory diseases, are they play synergistic roles or function at different time point or location? How TGF-β regulate Th17 and Treg cells？Can Th17 cells be used as a target for immune tolerance induction? All above questions will certainly be of continuing interest.