The osteoblast was cultured from cranium of fetal rat with enzymatic digestion method; bone marrow lymphocyte was cultured by density gradient centrifugation; two kinds of cells were irradiated with 137Cs γ-rays of 6 Gy; the groups were set as 0 Gy osteoblast+0 Gy lymphocyte(0OB0L), 0 Gy osteoblast+6 Gy lymphocyte(0OB6L), and 6 Gy osteoblast+6 Gy lymphocyte(6OB6L). After 6 h co-cultivation, the lymphocyte was co-cultured with normal osteoblast, and then the proliferation and differentiation of osteoblasts were detected by MTT and PNPP after 72 h, meanwhile, the mRNA expression of ALP, OCN, RANKL, and OPG of the osteoblast were detected after 2 h. The results suggest that radiated lymphocyte can inhibit the differentiation of osteoblast, down-regulate the mRNA expression of ALP and OCN, up-regulate the mRNA expression of RANKL and increase the ratio of RANKL/OPG. The radiated bone marrow lymphocyte not only suppresses the differentiation of osteoblast, but also affects the osteoclast by changing the ratio of RANKL/OPG.Cited