Five major role of TGF-β1 in osteoimmunity are shown. (1) TGF-β1 stimulates the proliferation of MSCs, and promotes their differentiation into chondrocytes. (2) TGF-β1 promotes osteoblast progenitor’s differentiation into osteoblast. (3) High concentration of TGF-β1 enhances osteoblast proliferation, and downregulates the expression of RANKL of osteoblast. (4) Low concentration of TGF-β1 promoted osteoclast maturation. (5) TGF-β keeps hematopoietic stem cells (HSC) in hibernation state.