Abstract: The behaviour and mechanism of the oxygen concentration difference macrocell were investigated for steel A₃ in the 3.5% NaCl solution by the self-made equipment.Three kinds of macrocell:O₂/N₂,O₂/air,N₂/air macrocells were formed.The i_g~t,E_g~t curves were recorded in the experiments.The coefficient of the macrocell corrosion effect, r=i_a^A/i_k^A, was proposed for calculation of the effect intensity of macrocell.The results show that the effect behavior of these macorcells was the same,and all macrocells accelerated the corrosion rate of the anodic metal in macrocells.The correlation between the value of coefficient r and the real intensity of macrocell was testified to be relatively satisfactory.