Strengthening of reinforced concrete beams with prestressed Fiber Reinforced Polymer (FRP) sheets showed to be efficient in enhancing the serviceability of the strengthened concrete beams. Yu et al.'s develop a mechanical prestressing device to prestress FRP sheets to strengthen reinforced concrete members. Yu et al. s experimental results showed that this device overcome the shortcomings associated by using the previously developed prestressing devices. In this research program Yu et al.'s mechanical device was modified to increase the efficiency of it by using light weight material to reduce the additional weight on the strengthened beams caused by the prestressing device in addition, the end anchorage plates were installed vertically rather than horizontally to extend the prestressed FRP sheets to the end regions of the member where high shear stress occurred. This modification in the end anchorage regions is efficient to reduce the possibility of the premature debonding failure. A numerical analysis by using ANSYS (R14.5) computer program was used to simulate the prestressing device. Numerical results showed that using aluminum material is efficient to produce a prestressing mechanical device. The maximum stresses and deflection that generated in the pretressing device upon prestressing Carbon Fiber Reinforced Polymer (CFRP) sheet (51% of the ultimate tensile stress in the CFRP) to strengthen 22.5m concrete beam are 46MPa and 83mm respectively. A theoretical formula was derived based on geometric relations of the prestressing system and the deformed CFRP sheet. It was used to predict the prestressing level in the CFRP sheet. Theoretical results showed that the percentage difference between Yu et al.'s formula and the derived formula is about 1%. This value changes based on the prestressing level in CFRP sheet and the length of the strengthened beam. The theoretical prediction of prestressing level in the CFRP sheet agreed well with the experimental results.