The anaerobic corrosion of mild steel in seawater was studied by atomic force microscopy (AFM). In the presence of sulfate reducing bacteria (SRB), corrosion was intensified and accelerated. A biofilm consists of heterogeneous microbial cells and extracellular polymeric substance with interstitial voids was observed on the surface of mild steel coupons. The greatest damage of steel occurred beneath the biofilm, in the form of pitting corrosion. The corrosion of steel can be quantified through section and bearing analyses. The depth of pits increased linearly with time whereas the volume of pits increased as t(2.83), the 2.83 power of time. Compared with a control experiment without SRB, the depth of pitting corrosion is about one order of magnitude higher. Weight loss estimates from AFM images are about one order of magnitude smaller than actual weight loss experimental results. The problems of AFM quantification of corrosion rate at extended stage of corrosion are discussed.