Biological elements have significant impact on lifetime prediction of marine carbon steel facilities. Microbe can produce pitting, crevice corrosion, selective dealloying and stress-oriented hydrogen-induce cracking, which accelerates both localized and average corrosion rates of carbon steel. The formation of microbe films can also reduce the steel corrosion rate through inhibition of oxygen diffusion and depletion of oxygen in the electrolyte and metal/solution interface. A comparing study of the corrosion behaviors of 45 steel in natural seawater and in sterile seawater in tropic condition shows that in most of immerging periods, the bacterial activity at the interface accelerates the average corrosion rate of the steel. However, when the corrosion time is 28 d, the biofilms inhibit the corrosion of 45 steel. The species and contents of microbe significantly influence the corrosion behavior of the steel. The microbe in the corrosion product mainly consists of Pseudomonas, Vibrio, Crenothrixandleptothrix, Thiobacillus, and Sulfate-reducing bacteria. The content of aerobe does not change with increasing the immerging time, but the content of anaerobe increases with increasing immerging time. The regular change of the content of microbe with the immerging time leads to the different microbe corrosion mechanisms for 45 steel.