To evaluate the relationship between endometrial thickness during fresh in vitro fertilization (IVF) cycles and the clinical outcomes of subsequent frozen embryo transfer (FET) cycles. FET cycles using at least one morphological good-quality blastocyst conducted between 2012 and 2013 at a university-based reproductive center were reviewed retrospectively. Endometrial ultrasonographic characteristics were recorded both on the oocyte retrieval day and on the day of progesterone supplementation in FET cycles. Clinical pregnancy rate, spontaneous abortion rate, and live birth rate were analyzed. One thousand five hundred twelve FET cycles was included. The results showed that significant difference in endometrial thickness on day of oocyte retrieval (P=.03) was observed between the live birth group (n=844) and no live birth group (n=668), while no significant difference in FET endometrial thickness was found (P=.261) between the live birth group and no live birth group. For endometrial thickness on oocyte retrieval day, clinical pregnancy rate ranged from 50.0% among patients with an endometrial thickness of <6mm to 84.2% among patients with an endometrial thickness of >16mm, with live birth rate from 33.3% to 63.2%. Multiple logistic regression analysis of factors related to live birth indicated endometrial thickness on oocyte retrieval day was associated with improved live birth rate (OR was 1.069, 95% CI: 1.011-1.130, P=.019), while FET endometrial thickness did not contribute significantly to pregnancy outcomes following FET cycles. The ROC curves revealed the cut-off points of endometrial thickness on oocyte retrieval day was 8.75mm for live birth. Endometrial thickness during fresh IVF cycles was a better predictor of endometrial receptivity in subsequent FET cycles than FET cycle endometrial thickness. For those females with thin endometrium in fresh cycles, additional estradiol stimulation might be helpful for adequate endometrial development.