Background:Estrogen and progesterone are two crucial factors for endometrial preparation in frozen embryo transfer (FET) cycles. Studies assessing different forms of estradiol in FET have published already but literature lacks enough surveys on transdermal estrogen application in reproductive medicine.Objective:To investigate the effects of trans dermal estrogen (Oestrogel) on pregnancy rates in patients that candidate for FET cycle. Materials and Methods:In this randomized clinical trial, 100 women undergoing FET cycles referred to Imam Khomeeini Hospital were enrolled in two groups, randomly. Group I received 8 mg/day estradiol valerate (E2 tablet) orally and group II were treated with 6 mg/day transdermal oestrogel gel after suppression with gonadotropin releasing hormone agonist. In both groups medication were started in the first day of menstruation cycle and continued until endometrial thickness reached 8 mm. Pregnancy rates (chemical, clinical, and ongoing), abortion rate, live birth rate, and frequency of complications were compared between two groups. Results: Chemical and clinical pregnancy rates were not significantly different between two groups (p=0.384). The abortion rate was significantly lower in group II than group I (p=0.035). Ongoing pregnancy and the live birth rates were significantly higher in group II (p=0.035). The rate of complication was not different in two groups.Conclusion:Oestrogel seems to enhance ongoing pregnancy and live birth rates in comparison to estradiol valerate tablet.