Abstract: Microgravity (MG) results in reduction of bone formation. Bone formation is bound up with osteogenic differentiation from human mesenchymal stem cells (hMSC) in bone marrow. We simulated MG to determine its effects on osteogenesis of hMSC and used activators or inhibitors of signaling factors to modulate osteogenic differentiation. Under osteogenic induction, MG reduceds osteogenic differentiation of hMSC and decreaseds the expression of osteoblast gene markers. The expression of Runx2 was likewise inhibited. However, the expression of PPAR$\g$2 increased. MG also decreased phosphorylation of ERK, but increased phosphorylation of p38MAPK. SB203580, a p38MAPK inhibitor, was able to inhibit the phosphorylation of p38MAPK, but not able to reduce the expression of PPARγ2. Bone morphogenetic protein (BMP) increased the expression of Runx2. Fibroblast growth factor 2 (FGF2) increased the phosphorylation of ERK, but did not increase significantly the expression of osteoblast gene markers. The combination of BMP, FGF2 and SB203580 significantly reversed the effect of MG on osteogenic differentiation of hMSC. Our results suggests that simulated MG inhibits the osteogenic differentiation and increases the adipogenic differentiation of hMSC through different signaling pathways. Therefore the effect of MG on the differentiation of hMSC could be reversed by the mediation of signaling pathways.