This paper presents a careful theoretical investigation into interfacial stresses in damaged RC beams strengthened with externally bonded FRP plate. The model is based on equilibrium and deformations compatibility requirements in and all parts of the strengthened beam, i.e. the damaged concrete beam, the FRP plate and the adhesive layer. The theoretical predictions are compared with other existing solutions. This research is helpful for the understanding on mechanical behaviour of the interface and design of the FRP-damaged RC hybrid structures.