Phytoestrogens, especially Yak-kong or soybean-derived isoflavones have been traditionally used as a supplement of estrogen for preventing postmemopausal osteoporosis in oriental folk medicine. In our previous study, the treatment of Yak-kong and soybean increased estrogen receptor-a (ERa) expression and proliferation of MG-63 osteoblastic cells. In contrast, the increase of estrogen receptor-$eta$ (ER$eta$) expression in proliferating MG-63 cells with Yak-kong and soybean treatment was less pronounced, which suggested that ER$eta$ may play a role rather in the regulation of bone cell differentiation To determine the role of ER$eta$ in Yak-kong or soybean mediated regulation of bone cell differentiation, we established MG-63 cell lines stably expressing either ER$eta$ or antisense ER$eta$ RNAs. Increased expression of ER$eta$ did not affect ERa expression and proliferation of MG-63 cells. However, increased expression of ER$eta$ in MG-63 cells (ER$eta$-MG63 cells) selectively enhanced Yak-kong or soybean induced expression of osteoprotegerin (OPG), a novel soluble glycoprotein which is secreted from osteoblasts and mediates the signal for osteoclast differentiation. Inhibition of ER$eta$ expression by antisense ER$eta$ RNAs (As-ER$eta$-MG63) caused these cells to insensitize Yak-kong or soybean induced expression of OPG but increased MG-63 cell proliferation. Furthermore, the comparable effects between Yak-kong and the combined treatment of genistein and daidzein at $0.5{ imes}l0^{-8}$ M, which is a concentration of these two isoflavones similar to Yak-kong at 0.001 mg/mL, on OPG expression in ER$eta$-MG63 cell demonstrate that the enhanced expression of OPG with Yak-kong treatment is mediated by the synergistic effect of low leveled isoflavones in the extracts. Together, coupled with low level of ER expression in osteoclasts, our data demonstrate that ER$eta$ in osteoblasts plays an important role in Yak-kong and soybean mediated inhibition of osteoclast differentiation indirectly by enhancing the expression of OPG.