Abstract: To investigate the effect of O2 tension on sheep embryo development in vitro. The developmental ability and cell apoptosis of Sheep IVF (In vitro fertilization) embryo in low (5%) and high (20%) O2 tension environment were compared. The expression profiles of apoptotic related genes Bax and Bcl-2 in sheep IVF embryos were also detected by Real-time PCR. The results showed that: the apoptosis signal of the sheep IVF embryo was all commenced from the 16-cell stage under two O2 tension environment, there was no significant difference in the rate of apoptosis among respective developmental stages under two O2 tension environment, but the blastocyst development rate and cell number of blastocyst cultured in low O2 tension environment were significantly higher than those of high O2 tension environment (45.9%,134 and 14.6%,106, respectively P＜0.05), and the apoptosis index of blastocyst developed in low O2 tension environment was significantly lower than that of high O2 tension environment (0.053 and 0.066, respectively P＜0.05); As assessed by Real- time PCR, Bax and Bcl-2 were expressed in sheep IVF embryo at all stages developed in low O2 tension or high O2 tension environment, The expression of bcl-2 in sheep IVF blastocyst developed in low O2 tension environment was significantly higher than that of high O2 tension environment. Low O2 tension environment could inhibited the apoptosis of sheep IVF embryo, and should be more beneficial to the development of sheep embryo.