Plant lignans are converted by the intestinal microflora to the mammalian lignans enterodiol and enterolactone, which are associated with beneficial health effects in humans. The mammalian lignans enterodiol and enterolactone are believed to protect against certain diseases, e.g., breast and prostate cancer as well as coronary heart disease. In this study, the effects of enterodiol and enterolactone on osteoblastic differentiation were examined. It was found that enterolactone and enterodiol showed the same effects. They have biphasic effects on cell viability, alkaline phosphotase (ALP) activity, transcriptional level of osteonectin, and collagen I, showing induction at low doses and inhibition at high doses. They increased transcriptional levels of ALP, osteopontin, and osteocalcin in a dose-dependent manner. The difference may be related to the estrogenic and antiestrogenic character and multiple signaling transduction of phytoestrogen.