Oral contraceptive pretreatment (OCP) is commonly used to suppress cyst formation, to schedule the treatment plan, to induce endometrial bleed in oligomenorrhea patients, and to avoid ovarian hyperstimulation syndrome. The published studies on OCP have not been powered to assess the impact of OCP on likelihood of pregnancy. In this retrospective cohort study we examined if OCP benefit the clinical outcomes in a series of consecutive patients undergoing long GnRH-a down-regulation protocol with IVF/ICSI treatment between January 2011 and July 2012. The study included 3115 cycles with and 1079 cycles without oral contraceptive administrations. Compared with patients without OCP, patients with OCP required lower duration and dosage of gonadotropin stimulation; had a similar average number of oocytes retrieved, cancellation rate, and OHSS rate; but the patients with OCP had lower implantation rate, PR per transfer, and PR per cycle started. Theresults were similar regardless the level of ovarian reserve. This study indicates that Oral contraceptive may have unfavorable impact on pregnancy in long GnRH-a down-regulation with IVF/ICSI cycles.