During the early stage of incubation of chicken eggs, different CO2 concentrations play an important role in the embryonic and postnatal growth. However, the mechanism involved in this process is still not well clarified. In this study, chicken eggs were incubated under different CO2 concentration levels (0.03-0.05, 2, 5 and 10%) during the 1st 4 days of incubation and measured of several embryo growth parameters. It was then found that higher CO2 concentration significantly played down the albumen pH value after 2 days of incubation and the embryo weight and length, diameter of area vasculosa of yolk sac and number of somite all decreased with the increasing of CO2 level. Compared with the mortality (3.23%) and abnormality (0%) in the control group, the corresponding percentages in three treatment groups were much higher with the lowest at 17 and 29%, respectively. Taking the two sides together, it might be well concluded that high CO2 concentration during the 1st 4 days of egg incubation inhibited chicken embryonic development through the lowering of albumen pH value.