This study was carried out to efficiently use the ultrarapid freezing method in the cryopreservation of mouse ova. For this, the effects of dehydration method, oval vigour and controlling method on post-thawing viability were investigated. Fresh mouse ova were dehydrated in mPBS with 3.5M DMSO and /or 0.25M sucrose, and directly immersed in L for ultrarapidly freezing. The frozen ova were thawed at 37, rehydrated in mPBS with 0.25M sucrose, and then repeatedly washed in HAM's Fl0 before evaluating the morphological normality of frozen-thawed ova. The results obtained showed that there was difference between treatments in a experiment. 1) The post-thawing viability of ova dehydrated in multi-step (48.413.8%) was higher than that of ova in two-step (40.914.0%). 2) The post-thawing viability of fertilized ova (8714.0%) was significantly(p<0.0l) higher than that of unfertilized ova (5.45.4%). 3) The post-thawing viability of ova dehydrated and rehydrated using a cooling machine (95.84.2%) was significantly(p<0.05) higher than that on ice(84.19.9). In conclusion, in order to efficiently cryopreserve ova in vitro with ultrarapidly freezing method, highly viable embryos should be selected, heavy osmotic shock to the dehydrating ova should be avoided, and embryos in high osmotic condition were dehydrated and rehydrated in a constantly low temperature.