Adding nanoparticles into cryoprotectant may be an important means of cryopreservation in future. The oocytes were cryopreserved using cryoprotectant containing hydroxyapatite (HA)

and the morphology of crystal and oocytes in the process of crystallization

recrystallization and thawing were observed using cryomicroscope system. In the mean time

the temperatures were recorded when every phase change occurred. It is found that cryoprotectant with 0.05%HA recrystallized implicitly with narrow danger zone during rewarming and hence little harm was done to oocytes. In contrast

the group without any HA nanoparticles tended to get harmed. In conclusion

the mechanism that nano-cryoprotectant promoted the survival rate of oocytes was that nanoparticles hindered the recrystallization of cryoprotectant in the process of rewarming.