To evaluate the effect of estrogen on its ability to restore the bone mass and bone quality in ovariectomized rats by examining the changes of bone morphology and histomorphometry, 3month-old rats were divided randomly into 4 groups: normal control, ovariectomized (OVX), shamoperated (Sham-O) and OVX plus estrogen (OVX+E2). Treatment initiated from the day 8 weeks after operation and continued for 12 weeks. Bone morphology and histomorphometry were examined afterwards. Results showed that comparing to control group, the trabecular bone in OVX appeared thinner and reduced in the amount. The connectivity between trabecula was decreased and the structure disordered. The free-end of trabecula was increased. The cavity of bone marrow enlarged. After treatment with estrogen, above changes improved remarkably by different degree, although did not reach the normal face. The bone histomorphometry results damonstrated that estrogen treatment increased bone mass and the amount of trabecula by 129% and 132% respectively (P＜0. 05). The activity of bone resorption decreased significantly and the rate of bone formation increased to 203 %. These results suggest that treatment of ovariectomized rats with estrogen can not only increase bone mass, also improve the bone structure and enhance the property of bone mechanics.