Objective: To explore the relationship between endometrial thickness and clinical pregnancy outcomes in frozen-thawed embryo transfer cycles. Methods: A prospective study was performed for 1 475 frozen-thawed embryo transfer cycles at Peking University People's Hospital from January 2014 to December 2015. The patients were divided into different groups according to endometrial thickness of ovulation day in natural menstrual cycles or endometrial transformation day in hormone replacement cycles;patients with thin endometrium were enndometrial thickness ≤6 mm. Then the clinical pregnancy outcomes including clinical pregnancy rate, embryo implantation rate, abortion rate, multiple birth rate and live birth rate were analyzed. Results: In all, 1 475 frozen-thawed embryo transfer cycles were analyzed. The mean age of patients was (32.5±3.9) years old and mean endometrial thickness was (9.2±1.9) mm, and mean number of embryos was 2.03±0.37. The study included 518 (35.1%) natural menstrual cycles and 957 (64.9%) hormone replacement cycles. The number of embryo-transfer cycles and blastocyst-transfer cycles were respectively 700 (47.5%) and 775 (52.5%) . The overall clinical pregnancy rate, embryo implantation rate, abortion rate, multiple birth rate and live birth rate were 54.4%, 35.7%, 23.3%, 24.1%, 43.9%, respectively. The ectopic pregnancy rate in the study was 0.6%. In patients with thin endometrium,there were significant differences in 2 pronucleus count (P=0.016) and available embryo count (P=0.024) between cycles that resulted in pregnancy and those that did not;besides, the use of sildenafil and growth hormone did not improve pregnancy outcomes in patients with thin endometrium (P=0.183, P=0.400) . The clinical pregnancy rate, embryo implantation rate and live birth rate of embryo-transfer and blastocyst-transfer were similar in patients with thin endometrium (all P>0.05) . Conclusions: Patients with thin endometrium have poor pregnancy outcomes. The clinical pregnancy rate, embryo implantation rate and live birth rate of embryo-transfer and blastocyst-transfer are similar in patients with thin endometrium. Compared thin endometrium and non-thin endometrium patients, the clinical pregnancy rate and live birth rate of blastocysts have more substantial decline than those of embryos. Improving the quality of embryo could improve the pregnancy outcome of patients with thin endometrium. Sildenafil and growth hormone could not improve pregnancy outcome in patients with thin endometrium.