Objectives: This study was conducted to evaluate the inhibitory effect of polygoni cuspidati radix (PCR) extract on osteoclast differentiation. Methods: MTT-assay was performed to estimate cytotoxicity of PCR extract in BMMs stimulated with RANKL. Tartrate resistant acid phosphatase (TRAP) staining, TRAP activity and RT-PCR were performed to know the inhibitory effect on osteoclast differentiation. actin ring formation were analysed to observe the effect of PCR extract. Results: PCR decreased the number of TRAP positive cells and TRAP activities in BMMs stimulated with RANKL and M-CSF. PCR restrained the formation of actin ring. PCR down regulated the induction of NFATc1, c-Fos, TRAP and OSCAR by RANKL. PCR inhibited NF-${kappa}B$ activity by inducing degradation of $I{kappa}B{alpha}$. Conclusions: We suggest that PCR Extracts can be an effective therapeutic agent on osteoclast differentiation caused by diseases such as osteoporosis.