〔Abstract〕 Objective To investigate the influence of gonadotropin releasing hormone agonist(GnRH-a) on pregnancy outcomes in frozen-thawed embryo transfer cycles for patients with endometriosis and infertility in normal serum CA125. Methods A retrospective analysis was performed on 146 FET cycles in patients who didn′t accept GnRH-a with EMs in normal serum CA125 before FET(0-35 U/ml), the ROC curve and the best statistical cutoff values were obtained. According to the best statistical cutoff values and whether GnRH-a was applied before FET,228 patients were divided into Group A( CA125 was 0-16. 79 U/ml without GnRH-a),group B( CA125 was16. 79-35 U/ml without GnRH-a) and Group C( CA125 was 16. 79-35 U/ml with GnRH-a),and the general condition and pregnancy outcome were compared among the three groups. Results There were no statistically significant differences between the three groups in age,years of infertility,body mass index,number of transplanted embryos and high quality embryo rate. There were statistically significant differences in embryo implantation rate and clinical pregnancy rate between Group B,C and Group A,and there were statistically significant differences in embryo implantation rate and clinical pregnancy rate between group B and Group C( P<0. 05). There were no statistically significant differences in endometrial thickness,ectopic pregnancy rate and live birth rate among the three groups. Conclusion Serum CA125≥16. 79 U/ml before FET in EMs patients can predict the adverse pregnancy outcome,and applying GnRH-a before FET can improve the adverse pregnancy outcome in EMs patients.