Abstract: Experimental study on 9 corroded reinforcement concrete beams with cantilevers at both supports in atmospheric environment is carried out. It is easier for the stirrup to corrode than longitudinal steel bars. Plane sections of corroded R.C. beams before bending remain plane after bending when the corrosion rate is lower than 5%. There is no obvious yielding point in corroded R.C. beams. The decrease of bonding capacity between concrete and corroded re-bars induces the decrease of beam rigidity and increase of average distance between cracks. The loss of load carrying capacity of corroded R.C. beams with more longitudinal cracks and wider cracks is greater than those with less longitudinal cracks and narrower cracks. The loss of the load carrying capacity of corroded R C beams in the positive-moment zone and the negative-moment zone is about 1.7 to 2.5 times and one times of the corrosion rate.