Open recirculating cooling water system is one of main systems to cool process equipment. Cooling water system management is to keep water quality. Water quality is affected by the quality of raw water and the atmosphere environment. Therefore water quality monitoring is important. The use of an electrochemical noise (EN) method for corrosion monitoring in process plants is effective because it is highly responsive and more convenient for on-site measurement than other corrosion monitoring methods. In this study the use of the EN instrument is demonstrated for monitoring water quality in an open recirculating cooling water system.