Abstract: Carbonization and chloride diffusion are the two most important factors which affect durability of concrete in the aspect of environment. The process of carbonization changes the properties of concrete and affects chloride diffusion. The concrete samples were cured under the same conditions. The concrete samples were carbonized to different degrees in the carbonization box with the same conditions inside. Current and electric quantity method has been used to test the chloride diffusion coefficients in the concrete samples with different carbonization depths, and to analyze the effects of carbonization on the chloridion diffusion and the probable reasons for it when other conditions are the same. Experiment results show that the chloride diffusion coefficients will grow when the carbonization depth is increased. Further analysis results show that the carbonization has both positive and negative effects on the chloride diffusion. As the negative effect， carbonization increases the degree of density in concrete which prevents the chloride diffusion. As the positive effect， carbonization increases the amount of capillary porosity and content of free chloridion in concrete which promotes chloride diffusion. The effects given by carbonization on the chloride diffusion depends on the aspect which is more dominant in the practical situation.