Recent studies investigate the behavior of external post-tensioning tendons to enhance the load-carrying capacity and increase the services life of the structures. This study aims to predict the optimum strengthening for a composite beam with prestressed carbon fiber reinforced polymer plate to enhance the ultimate tensile capacity and reduce ductility. The 3-D finite element (FE) method was used to investigate the flexural behavior of the strengthened composite beam. Two experimental results from the literature are used to validate the FEM results. Three parametric studies were investigated; the effect of strengthening with CFRP tendons on various plate materials, the effect of strengthening with CFRP plate on various plate materials, and the effect of tendons and plates length on the beam capacity.