In order to prove the assumption that the corrosion rate of steel in concrete is controlled by the diffusion rate of oxygen, the effect of Icorr on the degree of the pore saturation in concrete at a given temperature was investigated by monitoring corrosion current. A comparison test of the corrosion rates of passive steel, active steel in completely water saturated concrete and active steel in atmosphere environment was conducted. The phase component of corrosion products of steel bars before and after the specimens were immersed into water was measured with the X-ray diffraction analysis technique. The research has indicated that the corrosion rate will decrease when concrete dries (the pores are more open and the pathway for oxygen is higher). After the corrosion starts, FeOOH in corrosion layer can act as depolarizer instead of oxygen, so the corrosion rate is still very high even if concrete is completely water saturated. The oxygen is one of the factors that influence the corrosion rate but is not the factor controlling the corrosion process of steel in concrete. Therefore, all predictions based on the diffusion rate of oxygen through concrete cover must be regarded as not reliable.