Objective: To explore the better protocol for endometrial preparation for frozen-thawed embryo transfer (FET) in unexplained repeated implantation failure(RIF) patients.Methods: 140 cycles of FET in unexplained RIF patients in our center from Jan 2014 to Dec 2016 were prospectively analyzed.The data were used to compare two different endometrial preparation protocols,natural (NC-FET,n=-91) versus arti ficial (AC-FET,n=49) cycles,for FET in women who have experienced unexplained RIF compared with infertile women in the first FET cycles (NC-IFC,n=107).Results:There were no significant differences among three groups in age,body mass index (BMI),infertility duration,the number and quality of thawed embryo,estradiol level,endometrial thickness and endometrial types on the day of endometrial transformation and the early miscarriage rate (P>0.05).Implantation rate and clinical pregnancy rate of NC-IFC were significantly higher than those of NC-FET and AC-FET (P<0.01),and Implantation rate and clinical pregnancy rates of NC-FET were higher than those of AC-FET (P<0.05).Conclusions: Natural cycles for endometrial preparation for FET in RIF patients may obtain better pregnancy outcomes,which indicates superiority of natural over artificial cycles in unexplained RIF patients undergoing FET.