Reinforced concrete (RC) beams strengthened with fiber reinforced polymer (FRP) sheets may fail due to debonding failure. In such cases, the FRP sheet detaches from the RC beam before real damages are inflicted on the beam. In this paper, a procedure is developed based on smeared cracks approach for simulating the debonding process in FRP strengthened RC beams within the framework of finite element. For this purpose, the challenges facing the simulation of debonding mechanism are initially studied and a method is proposed in a second stage for combating these problems using the cohesive elements available in the ABAQUS software. The validity of the proposed method is then tested by modeling four beams from those reported in the literature and by comparing the results with the experimental ones. Given the acceptable agreement observed between the experimental and numerical simulation results, the method is claimed to be valid and practicable. In a later section of the present paper, the proposed method will be used to investigate the effects of length and width of the strengthening sheet on beam’s behavior and its failure mechanism. The results of the present study have revealed that longer FRP sheets increase load carrying capacity and mid-span displacement of strengthened RC beams.