Frizzled 4 (FZD4) is an important receptor for Wnt proteins that stimulate several downstream signaling pathways. It has been known that the FZD4-Wnt interaction is involved in many types of cancers.FZD4 knockdown inhibited the mechanical stimuli-induced osteogenesis and JNK activity. More importantly, we found an activating effect of WNT5A and FZD4 that regulated bone formation in response to hindlimb unloading in mice, and pretreatment with WNT5A or activation of the expression of FZD4 partly rescued the osteoporosis caused by mechanical unloading.