Abstract: Objective：To compare the clinical outcomes of frozen-thawed single blastocyst transfer, on the different frozen day and with the different blastocyst score. Methods: This retrospective analysis included 1 675 cycles of frozen-thawed single blastocyst transfer in our IVF center from January 2008 to December 2015. Those cycles were divided into six groups, according to the frozen day and the blastocyst score. They are group D5BB and above, group D5BC/CB, group D5CC, group D6BB and above, group D6BC/CB, group D6CC. The clinical outcomes were compared among groups. Results: There were no significant differences in those rates of monocyesis, gemellary pregnancy, ectopic pregnancy, first trimester abortion, preterm birth, full-term birth, and low birth weight infant among six groups（all P＞0.05）. The rates of clinical pregnancy, late trimester abortion and live birth were significantly different （both P＜0.05）. When compared between any two groups, there were no significant differences in the rates of clinical pregnancy and live birth between group D5 blastocysts and group D6BB, or between group D5CC and group D6 blastocysts (all P＞0.05). Interestingly, the rates of clinical pregnancy and live birth were significantly increased in the group D5BC/CB when compared with the group D6BC/CB and group D6CC（all P＜0.05）. Conclusions: In those frozen-thawed cycles of single blastocyst transfer, there are good clinical outcomes when do the transfers on D5 with high and mediate-grade blastocysts, and on D6 with high-grade blastocysts.

Key words: Embryo transfer, Pregnancy, multiple, Pregnancy outcome, Embryonic development, The blastocyst score