One of the mediators of osteoclast differentiation is receptor activator of nuclear factor κB ligand (RANKL), which is produced by osteoblasts. Binding of RANKL to its receptor, RANK, activates several signaling pathways, including those involving mitogen-activated protein kinases (MAPKs), nuclear factor κB (NF-κB), nuclear factor of activated T cells c1 (NFATc1) and Ca²⁺-calcineurin. In the present study, we found that tetrandrine, a bisbenzylisoquinoline alkaloid extracted from the root of Stephania tetrandra S. MOORE, significantly ameliorated the decrease of bone mass in sciatic-neurectomized osteoporosis model mice. It appears that tetrandrine acts directly on osteoclast precursors, since tetrandrine inhibited osteoclast differentiation not only in mouse bone marrow cells, but also in monocultures of murine macrophage RAW 264.7 cells without osteoblasts. Tetrandrine suppressed RANKL-induced amplification of NFATc1, a master regulator of osteoclast differentiation. However, it did not affect other signaling molecules such as MAPKs and NF-κB. These results suggest that tetrandrine is a candidate for the treatment of bone-destructive diseases, or at least a suitable lead compound for further development.