Abstract The combination of phosphorus and zinc is mainly used as the carbon steel corrosion inhibitor for cooling water systems in the industrial plants such as petroleum refining or petrochemical plants. Moreover, treatments with high-concentration phosphorous and zinc are also applied to inhibit initial corrosions when the industrial plants are started up. Recently in some countries such as China, using phosphorous is prohibited by the wastewater regulation because of increasing conscious for environment. Therefore, non-phosphorus treatments for cooling water systems are highly required. In this presentation, new developed non-phosphorus treatments (initial and maintenance treatments) are introduced based on the laboratory and field tests. (1) Initial treatment: The combination of a specific organic acid and zinc showed the same inhibition ability as that of high-concentration phosphorus and zinc treatments. (2) Maintenance treatment: The combination of low-molecular weight polymers and zinc showed the same inhibition ability as that of phosphorus and zinc treatments. It was assumed that the polymers improve the corrosion inhibition abilities by dispersing zinc effectively. Introduction For cooling water treatment chemicals, environmentally friendly chemicals should be developed continuously because huge amount of industrial water are used for cooling water systems and it is necessary to concern effect for environment. As carbon steel corrosion inhibitors for cooling water systems in the industrial plants such as petroleum refining or petrochemical plants, chromates treatments were prohibited. The combination of phosphorus and zinc is mainly used because the enforcement of various regulations limit the applicable chemical compositions. 1 Now it is necessary to develop non-phosphorus treatments because regulations of phosphorus discharge from cooling water is expanding globally to prevent ecosystem in environmental water from eutrophication. Especially, China and several parts of USA strictly regulate phosphorus discharge. Therefore, the using phosphorus for cooling water treatment is limited in these areas. Aside from regulations, using phosphorous for corrosion inhibitor in cooling water systems causes undesired issues such as the formation of calcium phosphate scales, increasing cyanobacteria and algae. 1 By using phosphorous as corrosion inhibitors, it is necessary to use scale and slim inhibitors.