Objective To observe the effects of calcitonin on osteoclast activity and expressions of osteoprotegrin (OPG) and receptor activator of NF-?B ligand (RANKL) in vitro. Methods Osteoclasts and their precursors were mechanically isolated from long bones of SD rats and incubated on bone sheet in MEM with different concentration of calcitonin. Number, morphology of the osteoclasts were detected with tartrate-resistance acid phosphatase (TRAP) staining. Image-Pro Plus counted the resorption pits and areas on slices, stained by toludine blue. RT-PCR was used to examine the expressions of OPG and RANKL of osteoclast. Results Number of osteoclasts were decreased with increasing CT concentrations. After incubation for 5 days, resorption pits were decreased in a concentration-dependent manner (P