Objective To investigate the effect of low oxygen tension (7.5% O2) on human embryo development in vitro and outcome of women undergoing IVF-ET. Methods The data of 280 routine IVF cycles with fresh embryo transfer on day3 conducted in the 2nd People's Hospital of Yunnan Province from February 2012 to August 2013 were retrospectively analyzed. Two groups, 7.5%O2 low oxygen tension (n=120) and 20%O2 high oxygen tension (n=160),were setup for early embryo (D1-D3) development in vitro. Results Good quality embryo on day 3 in low oxygen tension group showed morphologically appearance of blastomere mutually clinging to each other,a characteristic of slight compaction. Higher rate of good quality embryo (66.22%V. 47.54%) and of clinical pregnancy (53.33%v. 39.22%) were found in the low oxygen tension group. There were no significantly differences in rates of fertilization,cleavage and embryo utilization respectively between the two groups ( all>0. 05) . Conclusion High quality early stage human embryos can be developed in vitro in low oxygen tension, and may improve clinic outcome of IVF.