Organic inhibitor, in general, reveals the different inhibition mechanism to that of anodic inhibitor. When triethanolamine or nitrite added separately to water for inhibiting corrosion of ductile cast iron, huge amounts of inhibitor should be needed because the corrosion inhibitors have to overcome the galvanic corrosion between graphite and matrix. This work used ductile cast iron and corrosion properties were evaluated in a tap water with/without inhibitors. Corrosion rate was measured using chemical immersion test and also electrochemical methods including anodic polarization test were performed. Inhibited surface was analyzed using EPMA and XPS. Test solutions were analyzed using a FT-IR measurement. When triethanolamine and nitrite coexist in water, synergistic effect built up and thus their mixed concentration was reduced to about 1/30 times than single addition. This work focused on synergistic effect by nitrite and triethanolamine and its mechanism was newly proposed (This work was supported by the Nuclear Power Core Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131520000100).).