ABSTRACT Microbiologically Influenced Corrosion (MIC) has been the subject of extensive studies. Some of the bacteria are capable of sensing surfaces. Contacting the surface initiates a complex differentiation program resulting in e.g. synthesis of alginate. Metal surfaces are rapidly colonized by microorganisms in contact with natural or industrial aquatic environments, giving rise to a complex and strongly adhering microbial community, termed as biofilm. The biofilm accumulation not only protects microbial cells from the external environment, but also it is detrimental to the underlying substratum thereby causing physical degradation or biodeterioration of the metal surface. In order to circumvent this problem, numerous biocides have been tried, but unfortunately have failed to produce the expected outcomes. An increased dose of biocide may, or may not, succeed in overcoming the protection provided by this polysaccharide covering because these polymers restrict the permeability of the biofilms to most biocides. INTRODUCTION OWING TO its economic and environmental importance, especially for the oil and gas industry, Microbiologically Influenced Corrosion (MIC) has been the subject of extensive studies for the past five decades and several models have been proposed to explain mechanisms governing biocorrosion [1]. Corrosion is a leading cause of pipe failure and is a main component of the operating and maintenance costs of gas industry pipelines. Quantifying the cost of corrosion generally, and more specifically the cost associated with MIC, in the gas industry is not easily done and is controversial. The annual cost of iron corrosion in the United States is 276 billion dollars in 2010. Iron materials are corroded by not only pure physicochemical reactions but also metabolic activities of microorganisms. Corrosion of iron materials in this manner A STUDY OF MICROBIAL INFLUENCED CORROSION IN OIL AND GAS INDUSTRY S.A.Alavi Department of Chemical engineering Azad University Science and Research Branch of Tehran Mehdi Yari Department of Metallurgy and Materials Azad University Science and Research Branch of Tehran Hamidreza Mansouri* Azad University Science and Research Branch of Tehran Parsian Gas Refinery Company Hossein Esmaili Parsian Gas Refinery Compony * Fax: +987825230535 Tel: +989171825738 Email: hamidreza2010@gmail.com