Microbial contamination and corrosion rate of a cooling water system were analyzed three times in April, July, and October 2014. Microbial analyses and Biocide efficacy test were performed according to NACE standard TM 0194 and ASTM E645 standard, respectively. Weight loss measurementswere performed to determine the corrosion rate of simple carbon steel in water samples. General heterotrophic bacteria (GHB) and sulfatereducing bacteria (SRB) were observed in all water samples. Highest microbial contamination was observed in July which GHB and SRB counts were 106 and 102 per milliliter of water, respectively. Corrosion rate of April, July, and October samples were 3.5, 15, and 4.1 mpy that reduced to 1.7, 5.3,and 2.5 mpy respectively, after biocide application. The July sample required at least 30 ppm biocide, while the April and October samples, required 20 ppm biocide for effective results. The results not only demonstrated the importance of microbiologically influenced corrosion in the studied cooling water system but also revealed the importance of monthly biocide optimization in a particular system.