Abstract: This paper is concerned with the shear strength of simply supported reinforced concrete deep beams, based on plastic theory of concrete and upper bound method. The materials are assumed to be rigid-perfectly plastic. Modified Coulomb failure criteria with tension cut-off are adopted to predict yielding of concrete. The shape of the yield lines as well as the energy dissipating in the concrete and rebar are discussed. Based on the conservation of energy, the work done by external loads on a deep beam equals the internal energy dissipated in the concrete and reinforcement on the yield line, an implicit function of load factor is obtained. Minimizing the implicit function, the limit load for shear failure is found.