The application of fiber-reinforced polymer (FRP) materials for structural strengthening of concrete members increased due to their superior properties. Among different FRP strengthening systems, externally bonded reinforcement (EBR) technique is the most common method to strengthen reinforced concrete structures. In recent years, externally bonded reinforcement on grooves (EBROG) has been proposed as an alternative to conventional methods which is more efficient compared to EBR method. Strength efficiency of FRP plates/sheets in nonprestreed strengthened beams can be as low as 10% due to occurrence of debonding phenomenon which can increase the strengthening costs considerably. Furthermore, by strengthening a beam with unstressed laminates, FRP materials can only bear the additive loads and they are not efficient in carrying the existing loads that are a large portion of total loads in a member. Finally, by FRP strengthening, the stiffness enhancement is relatively small, so the serviceability of RC beams is not greatly affected by FRP repair. Better utilization of FRP material can be obtained by prestressing. Because by prestressing, not only premature debonding failure can be overcome, but also the materials contribute to the load carrying capacity under both service and ultimate conditions. Using prestressing thechnique also brings more advantages for strengthening a beam including better distribution of the cracks and decrease in crack width, unloading of the internal reinforcing steel. In this study, the flexural behavior of reinforced concrete (RC) beams strengthened with prestressed externally bonded reinforcement on grooves (EBROG) carbon-fiber-reinforced polymer (CFRP) plates was investigated both experimentally and analytically. A four point loading test was performed on 7 RC beams with cross-sections of 200×300 mm and length of 2600 mm. One beam reserved as reference and the others were strengthened with FRP. The effect of different parameters included the prestress level, using anchorages and the strengthening technique were investigated. The experimental results indicate that prestressed technique can be used to postpone the debonding and increase the strength efficiency of FRP plates. The results also showed that the cracking load and flexural stiffness of the prestressed beams were significantly increased. In addition, the maximum enhancement in ultimate load-carrying capacity of prestressed strengthened beams compared to the non-prestressed beam was 20%. It is also concluded that the prestressed CFRP strengthening method reduce deflections and crack width. Whereas expensive mechanical anchorages are needed for transferring the prestressing stresses to the beam in EBR method, the results indicated that the EBROG method can transfer the prestressing stresses to the beam without any end anchorage which can reduce the strengthening costs significantly. In the specimens strengthened with EBROG method, the flexural behaviors included the load carrying capacity, ductility and the debonding strain of CFRP plate enhance by 18, 53 and 36 % compared to EBR method, respectively. Keywords: Flexural strengthening, prestressed.