Several kinds of pipes in a nuclear power plant were buried in the soil. In order to control the corrosion problems, cathodic protection has been applied to protect the corrosion process in the outer surface of pipes and several corrosion inhibitors have been used to minimize the corrosion in the inner surface of the pipes. This work focused on the corrosion monitoring of the outside of coal tar-coated pipes buried in a nuclear power plant using a FEM method and the corrosion inhibition of the inside of the pipes using mixed inhibitors. The pipes buried in a nuclear power plant were ductile cast iron and these have been suffered under very high cathodic protection condition beyond the appropriate condition. However, cathodic potential measured in the site reveals the non-protected status. Converting from 3D CAD data of the power plant to appropriate type for a FEM simulation was performed and cathodic potential under the applied voltage and current was calculated using the primary and secondary current distribution physics conditions. FEM simulation for coal tar-coated pipe without defects showed the over-protection condition if the pipes are well-coated. On the other hand, we compared the different inhibition mechanisms of organic inhibitor with that of anodic inhibitor. When ethanolamines or nitrite or molybdate were added separately to tap water for inhibiting the corrosion of ductile cast iron, large amounts of inhibitor were needed. However, when mixed inhibitors were added, their amounts were greatly reduced and we found the synergistic effect and also proposed the new mechanism.