Abstract: Objective: To investigate the effects of letrozole( LE) on the ovulation in patients with polycystic ovary syndrome( PCOS). Methods: The patients with good follicular growth and nomal ovulation after taking ovulation stimulator for 44 cycles were divided into letrozole group and clomifene( CC) group. The LE group with 20 cycles took 5 mg of LE per day for 5 days at the fifth day of menstrual cycle or withdrawal bleeding by oral. The CC group with 24 cycles took 100 mg of CC per day for 5 days at the fifth day of menstrual cycle or withdrawal bleeding by oral. The clinical pregnancy rate and early pregnancy abortion rate of two groups were compared. Results: Compared with the CC group,the endometrial thickness and clinical pregnancy rate of LE group with human chorionic gonadotropin period was higher,its serum estradiol level was lower( P 0. 05 to P 0. 01). There were not statistical differences in dominant follicle number,serum luteinizing hormone level,serum progesterone,three line rate of endometrium and early pregnancy abortion rate between two groups( P 0. 05). Conclusions: The effect of LE on promoting ovulation is good,the development of follicle and endometrium is synchronous. The clinical pregnancy rate is high after promoting ovulation. The LE may be a promising promoting ovulation drug.