Abstract: Effect of microbe in urban sewage on corrosion resistance of mild carbon steel was studied by weight-loss,EIS and SEM methods.The results showed that microbe in urban sewage accelerated the corrosion rate of carbon steel.During different phases,the activity of microbe and the electrochemical process was different.At the early period,charge transfer was the control step instead of diffusion at the latter time when the inducement of pitting corrosion was accelerated.At last,charge transfer became the control step again with the microbe dying.The SEM observation of corrosion products indicated that the presence of microbe changed the type of corrosion.