The mechanism of the diagonal tension failure of the reinforced and prestressed concrete beams with the shear reinforcement was investigated. In the experimental program, static-reversed cyclic loading tests on half-scale post-tensioned precast concrete beams were conducted. Two experimental parameters-the shear span ratio and the shear reinforcement ratio-were selected. In the analytical program, the width of the primary shear crack and the failure mode using the fracture energy of concrete released at the shear cracking were predicted. The analytical crack angle, crack width, and failure mode were verified by the comparison with experimental results. The proposed analytical model proved effective to evaluate the primary shear crack width of the reinforced and prestressed concrete beams.