Osteoclasts are bone-resorbing multinucleated cells derived from the monocyte/macrophage lineage. The differentiation of osteoclasts are regulated by osteoblastic cells expressed RANKL, which is the most critical molecule for osteoclast differentiation. In this study, we found that water extracts of cuscuta inhibited RANKL-mediated osteoclast differentiation by direct action on bone marrow macrophages (BMMs) without cytotoxicity. In BMMs, water extracts of cuscuta inhibited the mRNA expression of c-Fos, NFATc1, TRAP, and OSCAR. Also, the protein expression of c-Fos and NFATc1 was inhibited by water extracts of cuscuta treatement. Water extracts of cuscuta inhibited the phosphorylation of p38, ERK, and JNK in BMMs treated with RANKL. However, water extracts of cuscuta did not inhibit RANKL-induced I-${kappa}B$ activation. Water extract of cuscuta failed to inhibit bone resorption by osteoclasts cultured on hydroxyapatite plates. These results suggest that cuscuta may be a promising drug for use against bone disorders such as osteoporosis and rheumatoid arthritis.