Abstract: Objective:To evaluate the effect of Cortex Mori extract on the activity of osteoclasts and osteoblasts. Method:The water extract(W),ethanol extract(E),purified extract(R and PA) from Cortex Mori were prepared by reflux extraction,macroporous resin and polyamide column chromatography. The content of flavonoids in Cortex Mori extract was determined by visible spectrophotometry. Rabbit bone marrow cells were induced by 1α,25-(OH)₂VitD₃ to obtain the osteoclasts. After an 8-day osteoclastic induction,the cells were stained using the tartrate-resistant acid phosphatase(TRAP)kit. TRAP-positive cells were counted to evaluate the effect of Cortex Mori extract on osteoclast formation. MC3T3-E1 Subclone 14 cells were cultured and the effect of Cortex Mori extract on cell proliferation was determined by MTT assay. Result:The content of flavonoids in Cortex Mori extract was as follows:PA(37.23%)>R(26.80%)>E(21.83%)>W(15.38%). Extract PA(10,20,50 mg/L),R(20,50 mg/L),E(10,20 mg/L)and W(20 mg/L)were found to exhibit a lower number of TRAP-positive cells compared to the control group(P<0.01 or P<0.05). MTT assay indicated that all of the extracts could promote the proliferation of MC3T3-E1 Subclone 14 cell compared with the control group(P<0.01),and the proliferation effect was enhanced with the increase of flavonoid content. Conclusion:The study indicated that Cortex Mori extract could inhibit osteoclast formation and promote the proliferation of osteoblasts. Presumably,flavonoids might be the important bioactive component of Cortex Mori.