Abstract: "The cryopreservation of expanded mouse blastocysts in pellets by vitrification was studied at 25℃ with EFS30 solution (contains 21 % ficoll，0.35mol/L sucrose and 30% ethylene glycol) as a cryoprotectant. The expanded blastocysts in group 1 were equilibrated in EFS30 solution for 1 min，then pluged into liquid nitrogen, while in group 2 they were pretreated with 10% ethylene glycol for 5 min and equilibrated in EFS 3 0 solution for 1 min，then pluged into liquid nitrogen. The development rate and the hatch rate of blasto-cysts in group 1 were 100% and 70% respectively after being thawed and cultured , while in group 2 they were 98% and 63% respectively. The development rates in group 1 and 2 didn’t differ significantly from that of control (P＞0.05 ). The percentages of pregnant dams and full - term pregnancy offsprings were 90% and 71% respectively in group 1， while in group 2 they were 81 % and 58% respectively .after the thawed expanded blastocysts were transplanted into pseudopregnant mice. There were no significant differences in the percentages of pregnant dams and full - term pregnancy offsprings between the cryop-reserved groups and the control (P＞0.05). "