Abstract： Objection To study the effect of letrozole on EM rat models and influence on the reproductive system. Methods Surgically transplanted autologous uterine tissues to ectopic site beside the uterines in rats were used as animal models to study endometriosis. 20 EM model rats were random divided into letrozole-treated group and saline solution-treated control group. The change of ectopic lesion volume in each group was compared before and after treatment. Apoptotic cells were assessed by the terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick-end labeling (TUNEL) assay, and the uterian horn and ovary were weighted and observed by optical microscopy to study the change of morphology. Results The volumes of the endometriotic tissues of letrozole group reduced more than that in control group[ ( 28. 75 ± 2.28 )mm3 vs ( 108.39 ±9. 98)mm3, P <0.01 ]. The apoptotic rate in letrozole group [ (5.52 ±2. 81 )% ]was higher than control group[ (2.11 ± 1.70)%, P <0. 01 ]. The ovarian weights in letrozole-treated group increased significantly[ (25.25 ± 9. 89) mg/100g vs ( 13. 10 ± 2. 70 ) mg/100g, P < 0. 01 ], arid the ovaries showed polycyst. The uterian weights in letrozole-treated group[ (41.46 ± 15.81 ) mg/100g vs (94. 81 ±18.00) mg/100g, P <0. 05 ] significantly decreased, and the endometriums presented atrophy. Conclusion Letrozole treated EM by means of increasing the apoptosis of the ectopic tissues. Letrozole would give ovarian over stimulating and the uterian weighting decreased as well as endometriums atrophy.