This study focuses on a crack mechanics model—an infinite center-cracked concrete panel with a steel bar across a crack loaded by uniform tensile stress, under which the slip between reinforcement and concrete is neglected and the force produced by the reinforcement is regarded as a centralized force to close the upper and lower surface of the crack. The critical sizes of the fracture process zone (FPZ) are obtained by the use of a power-exponent tensile-strain softening model under the maximum tensile stress criterion and the maximum tensile strain criterion. The results show that the critical sizes of fracture process zone at both crack tips decrease with the increasing steel bar area, but the distance between the reinforcement and the crack tip or the decreasing Poisson ratio increase with the increasing of the tensile-strain softening index.