The tibia of rat was irradiated at the dose of 30 Gy, and then the rats were killed at 2 and 12 weeks after radiation to collect the serum; the osteoblast was cultured from cranium of fetal rat with enzymatic digestion method, and then the proliferation and differentiation of osteoblasts were detected by MTT and PNPP after 72 h, and the cell cycle of osteoblasts were detected by flow cytometry, meanwhile, the mRNA expression of cyclinD, cyclinE, p21 and p53 of the osteoblast were detected after 24 h. The results showed that the serum of the radiated rats inhibits the proliferation of osteoblast and increases the number of osteoblast in G1 phase, while decreases the number of osteoblast in S phase and G2 phase. Serum from the radiated rats 2 weeks later down-regulates the mRNA expression of CyclinD and CyclinE, meanwhile the serum from the radiated rats 12 weeks later up-regulates the mRNA expression of p21 and p53 and down-regulate the mRNA expression of CyclinE. The results suggest that the serum of the radiated rat inhibits the proliferation of osteoblast by down-regulating the expression of CyclinD and CyclinE and up-regulating the expression of p21 and p53 which retards osteoblasts in G1 phase.Cited