Abstract: Objective: To explore the impact of embryo blastomere number at cryopreservation, and different cryopreservation time on day 2 （D2） or day 3 （D3） after in vitro fertilization，on the clinical outcomes of the frozen-thawed embryo transfer（FET）. Methods：This is a retrospective analysis on 417 FET cycles. The patients were respectively divided into different groups according to cryopreservation time after in vitro fertilization，embryo blastomere number and good-quality embryo number. The clinical outcomes of the embryo-transfer were compared. Results：The embryo survival rate and total blastomere survival rate in the group of the blastomere number exceeded 4-cells on D2 after frozen-thawed were significantly higher than those in the group of the blastomere number less than 4-cells（89.35% vs. 79.11%，42.01% vs. 30.22% respectively，P＜0.05）. Compared with the group of blastomere less than 6-cells on D3，the group of blastomere exceeded 6-cells on D3 had higher embryo survival rate and total blastomere survival rate（86.52% vs. 72.43%，41.13% vs. 31.07%，P＜0.01）. The good-quality embryo rate on D2（56.84%） was significantly higher than that on D3 （48.82%），while the clinical pregnancy rate on D3（38.52%） was significantly higher than that of D2 （27.61%）. There was no significant difference in the embryo survival rate and abortion rate between D2 and D3 （83.50% vs. 79.44%，10.81% vs. 10.09%，P＞0.05） after frozen-thawed embryo-transfer. More good-quality embryos were transferred，higher the clinical pregnancy rate and multiple pregnancy rate obtained（P＜0.05）. Conclusions：Cryopreservation of more than 4 blastomeres on D2 and more than 6 blastomeres on D3 could get better frozen-thawed effect. The clinical outcomes of D3 cryopreservation embryo after FET were superior to D2 embryo.

Key words: Embryo cryopreservation, Frozen-thawed embryo transfer, Survival rate, Pregnancy rate