Gene expression and protein synthesization of osteoprotegerin (OPG) and osteoclast differentiation factor (ODF) were studied in shear stress cultured osteoblasts applying parallel-plate flow chamber system. Shear stress of 0.5, 1.0, 1.5 N/m² in 24 h, and step-wise increased shear stress from 0.5 to 1.0 and then to 1.5 N/m² and each stress for 8 h, and stepwise decreased shear stress from 1.5 to 1.0 and then to 0.5 N/m² and each stress for 8 h were applied. The expression of OPG and ODF mRNA was detected by RT-PCR, and synthesization of their protein was detected by Western blot. The results showed that the shear stress of 1.0 and 1.5 N/m² induced more significant changes in OPG mRNA and ODF mRNA than 0.5 N/m² and there was no significant difference between 1.0 and 1.5 N/m2. No marked differences were observed between the effects of step-wise increased shear stress and 1.5 N/m² after 24 h, and the same was between the effects of step-wise decreased shear stress and 0.5 N/m² after 24 h. The shear stress of 1.0 N/m² increased OPG expression and inhibited ODF expression in 24 h both in mRNA and protein level. In all, the ratio of OPG and ODF was increased by shear stress, which reminds that bone resorption was inhibited by the fluid flow, and shear stimuli regulated the new balance of bone resorption and bone formation throught OPG/ODF route.