Abstract: Due to deterioration of bond strength caused by corrosion, cooperative working capacity between steel bar and concrete is degrading, which makes steel strain lag behind concrete tensile strain. When the member corroded heavily, bond stress between steel bar and concrete is almost lost, and concrete in the compression zone may be failure before tensile steel bar arrives its yielding strain, which may make the member form a surplus reinforcement beam due to bond failure test. Simulation and finite element analysis on simulated corroded unbonded R. C. beams are made to investigate factors affecting the level of steel stress and its affecting rules in the limit states without bond. The relation between steel stress and reinforcement index of unbonded R. C. beams is established, which can be used to evaluate the bearing capacity of heavily corroded beams, and which can also be helpful to study the degrading mechanism and calculating method on bearing capacity for ordinary corroded R. C. beams.