Many studies have demonstrated that endometrial scratch significantly increase pregnancy rate in patients undergoing in vitro fertilization and embryo transfer. However, only a limited number of studies have been conducted in frozen thawed transfer (FET) cycle. Nowadays, in FET cycles, endometrial scratch is mainly performed on two different time points: 1) luteal phase in the previous cycle; 2)early follicular phase in the current cycle. However, few studies have compared the effectiveness of those two different endometrial scratch methods performed in different timing. The aim of this study is to find the best timing of endometrial scratch in FET cycles.