A new corrosion inhibitor was developed for prolonging service life of a closed-loop cooling system. It consists of nitrite, organic polymer, etc. The chemical forms the corrosion protection layer on the surface of cooling pipe. It also increases the solubility of scale materials into the cooling water and prevents building-up of scale compounds. It is very effective in the closed cooling system. To compare the relative performance of the conventional inhibitors with the new one in high hardness water, process simulation tests were carried out. In this high hardness water the anti-corrosion performances of the conventional inhibitors turned out to be poor. However, the program with the new inhibitor showed a very low corrosion rate. The corrosion rate caused by the new inhibitor was less than 1.7 MDD in the field test.