Abstract: Objective To compare the effect of low and ambient oxygen concentration on early embryo development and pregnancy outcome after in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). Methods One hundred and ninety-five pregnant women after IVF and ICSI who underwent routine ovulation promotion in our center were randomly divided into low oxygen concentration group (n=101) and ambient oxygen concentration group (n=94). Their ova were cultured with oxygen a low concentration and at an ambient concentration, respectively, to observe their fertilization, embryo development and pregnant outcome. Results No significant difference was found in fertilization rate, multiple fertilization rate, D1 cleavage rate, D2 and D3 embryo scores, fresh implantation rate, frozen and thaw rate between the two groups (P> 0.05). The pregnant rate in fresh implantation cycle was higher in low oxygen group than in ambient oxygen group(χ²=3.963, P< 0.05). No significant difference was found in pregnant rate in thaw implantation cycle between the two groups (P< 0.05). Conclusion Embryo cultured at low oxygen concentration can improve the pregnancy outcome after IVF and ICSI, which should be further confirmed in random controlled study with a large sample.