Microbiologically influenced corrosion (MIC) is one of the most difficult forms of corrosion to identify and mitigate. One reason behind such difficulty is the mechanism of attack by microorganisms is normally unclear and complicated. Iron-oxidizing bacteria induced aerobic corrosion contributes to a significant portion of MIC. In this study corrosion behaviors of carbon steel (CS) C1010 and stainless steel (SS) 304 in the presence of an iron-oxidizing bacterial species: Acidithiobacillus ferrooxidans was examined. Results showed that A. ferrooxidans cells could accelerate C1010 corrosion at a rate of 3 to 6 times of that without the presence of A. ferrooxidans but showed no effect on SS304 corrosion. The accelerated corrosion rate of C1010 with A. ferrooxidans cells was due to the presence of Fe3+ which produced by the metabolism of A. ferrooxidans cells could oxidize Fe(0) in a high rate. The oxidization was un-detectable for SS304 and A. ferrooxidans cells died off due to the deficiency of Fe2+. Also the oxidization was found to occur without needing the cells to attach to C1010 surface.