Six to twelve weeks old ICR mice were used as besic study on cryopreservation of mouse embryo to estimate critical temperatures of mouse embryo during freezing, and to study survival rate of frozen mouse embryo transfered to recipients. Embryos cooled at 1℃/min to -50℃ or -60℃ before plunging into liquid nitrogen had best survival rates. When the embryos were cooled at 7℃/min to -10℃, 60% of embryos was survived, but the survival ;rate was decreased when temperature at plunging into liquid nitrogen was lowered. Generally morula stage embryo was more tolerable against cooling than 8-cell stage embryo. The survival rate of embryos which were cooled to -40 or -50℃ at rate of 1 or 7℃ per min. and plunged into LN₂ was higher in morula than in 8-cell embryos exposed both in DMSO and glycerol. When the embryos were transferred into recipients, implantation rate was 12.2% in frozen embryo and 23.8% in fresh embryo.