In most studies, deterioration of concrete due to carbonation or chloride penetration is investigated separately. However, the deterioration of concrete is normally caused by the combination. The purpose of this study is to investigate the interaction between carbonation and chloride penetration and their effects on concrete. This was examined experimentally under various boundary conditions. For concrete under the double condition, the risk of deterioration due to carbonation was not severe. However, it was found that the carbonation of concrete could significantly accelerate chloride penetration. As a result, chloride penetration in combination with carbonation can cause harmful deterioration of concrete.