Three-points-bending tests on H-shaped steel beams are carried out to examine increase of stiffness of the beams that the flange with decrease thickness caused by corrosion is strengthened by CFRP reinforcing method. The beams with total length of flange bonded CFRP show increase of stiffness. The beams with partially bonding CFRP show segregation fracture of CFRP. The strains on edge of CFRP are measured to clarify the fracture mechanism and the strengthening design method of partially bonding CFRP is proposed to increase the stiffness of the beam with corrosion.