Objectives : Achyranthis Bidentatae Radix (ABR) has been used for treating of many symptoms especially osteoporosis and rheumatoid arthritis. In this study, we determined the effects of water extract of ABR in RANKL (Receptor Activator for Nuclear Factor $kappa$ B Ligand)-induced osteoclast differentiation culture system. Methods : We assayed mRNA expression levels of NFATc1, c-Fos, TRAP, OSCAR, $FcR{gamma}$, DAP12 and GAPDH in bone marrow macrophages (BMMs) treated with ABR. The protein expression levels of NFATc1, c-Fos, MAPKs and $eta$-actin in cell lysates treated with ABR were analysed by Western blotting. In addition we determined the effects of water extract of ABR on LPS-induced bone-loss mouse. Results : Water extract of ABR showed remarkable inhibition on RANKL-treated osteoclast differentiation without cytotoxicity. ABR down-regulated the induction of c-Fos and NFATc1 by RANKL. ABR suppressed phosphorylation of JNK, p38 and I-${kappa}B$. ABR rescued bone erosion by LPS induction in vivo study. Conclusions : These results demonstrate that ABR may be a useful remedy for curing of bone-loss disease such as osteoporosis.