Abstract: Objective To observe the effect of the estrogen on the mRNA expression of osteoprotegerin (OPG), osteoclast differentiation factor (ODF) and macrophage colony stimulating factor (M-CSF) in bone tissue of ovariectomized rats, and investigate the possible pathway of estrogen in preventing and treating postmenopausal osteoporosis. Methods Thirty healthy adult SD rats were randomly divided into sham operation group, ovariectomized group and estrogen-treated group. All rats were ovariectomized except those in the sham operation group. Bone density of the L3-L6 vertebra was detected 12 weeks after the operation. the total RNA were extracted from the femur to examine mRNA expression of OPG, ODF and M-CSF by real-time PCR. Results Estrogen increased the bone density of the ovariectomized rat lumbar vertebra and up-regulated the expression of OPG, whereas down-regulated the expression of M-CSF and lowered ODF:OPG ratio. Conclusion The effect of estrogen in treating postmenopausal osteoporosis is closely correlated with the regulation of OPG and M-CSF expressions and ODF:OPG ratio.