The nearshore reinforced concrete (RC) structures in Taiwan have been investigated for over ten years. The field survey included concrete cover layer and core samples drilled. The tests included the chemical analysis, microstructure analysis, and chloride diffusion rate etc. The test data show that the content of Cl(superscript -) in the existing marine RC structures is very high. The main factor that controls the corrosion of steel in the RC structures is the penetration of Cl(superscript -).The test data show that the more nearshore the structures are, the more accurate distribution of chloride penetration in the concrete will be. For 50 years service life of steel in RC structures, a minimum depth of 11.4cm concrete cover layer with better quality control of concrete is required in Taiwan.