The aim of this study was to investigate the morphology, structure and resistance of the biofilm including mixed bacterial population formed on the galvanized steel surfaces in a model water circulation system by electrochemical methods and to determine the effect of biofilm formation on the corrosion behavior of galvanized steel. The model water circulation system simulating a real cooling tower was run by maintaining uninterrupted supply of water for 3 months. The resistance of biofilm formed on the surface of the galvanized steels, which were removed from the model water circulation system montly, was determined by using electrochemical impedance spectroscopy (EIS). Polarization curves for each coupons were drawn. The biofilm formation and corrosion products were investigated by using scanning electron microscopy (SEM). As a result, it was determined that the structure and resistance of biofilm formed on the galvanized steel surfaces was different in each exposure time and thus the corrosion behavior of galvanized steel surfaces was changed according to time.