The main purpose of this study was to evaluate the effect of supplementation of a soybean protein concentrate (20%) on bone mass and plasma lipid profile of adult female rats as well as to test the effectiveness of a soybean isoflavonoid extract (0.5%) in preventing bone loss due to ovariectomy. Results showed that isoflavones separated from the soybean extract contain a higher amount of isoflavonoid glycosides (daidzin and genestin) than the aglycones (diadzein and genistein). Data obtained revealed that both plasma total cholesterol and high density lipoprotein cholesterol (HDL-C) of rats were not significantly changed due to the presence of the soybean protein concentrate (20%) in the diet. On the other hand, there was a significant decrease in plasma low density lipoprotein cholesterol (LDL-C) in soy protein group. Lipid peroxidation was significantly reduced due to feeding soybean protein concentrate (20%) of the diet (approximately 30% reduction). Rats fed the soybean protein concentrate (20%) diet had significantly higher mean bone densities (10% increase) of right tibia as compared with control (protein-based diet, 20%). The ovariectomized rats fed a diet containing the isoflavonoid extract had significantly higher mean bone densities (15% increase) and bone calcium of right tibia than ovariectomized rats fed the control diet. In conclusion, the present study demonstrated that soybean protein and isoflavones could have the potential to reduce the risks of postmenopausal osteoporosis and cardiovascular diseases in such women.