To Investigate the mechanism of postmenopausal osteoporosis in the metabolism of bone organic matrix，the levels of type Ⅰ collagen mRNA in ovariectomized rats bone tissue were detected by RT-PCR. The activities of bone metalloproteinases(MMPs) were determined by Gel-SDS-PAGE.The amounts of type Ⅰ collagen in bone tissues were estimated by immuno-histochemistry. The expression levels of type Ⅰ collagen mRNA decreased about 26.3%，and the activities of pro-MMP-9 increased markedly in ovariectomized group compared with Sham group. Estrogen increased the levels of type Ⅰ collagen mRNA about 34.1% and obviously reduced the pro-MMP-9 activities compared with OVX group. Ovariectomizing in rats resulted in the decrease of type Ⅰ collagen mRNA expression and the increase of pro-MMP-9 activity，both contributed to the decrease of bone organic matrix and ovariectomized osteoprosis.