Sex steroid hormones play important roles in bone metabolism through the functions of its specific nuclear receptors, estrogen receptors(ERs) and androgen receptor (AR). From the results of analyses of bone tissue specific sex steroid hormone receptors knockout mice, it has been clarified that sex steroids can directly function in bone tissue as well as indirect endocrine influences. These recent researches may promote the investigations of novel strategies to treat osteoporosis, which could be induced by deficiency of sex steroid hormones in the old population regardless of gender.