This study investigates the effectiveness and feasibility of a prestressed carbon fiber-reinforced polymer (CFRP) system and prestressed glass fiber reinforced polymer (GFRP) system for strengthening reinforced concrete (RC) beams. The proposed prestressing system with a novel anchorage allows the utilization of the full capacity of the sheets. Five small-scale, six small scale and four large-scale concrete beams strengthened with different configuration of prestressed CFRP and GFRP sheets are tested under static loading conditions up to failure. The main parameters considered include the level of prestressing applied, ranging from 23% to 60% of the tensile strength of the CFRP sheets, using of mechanical anchorages at both ends of the CFRP sheets. Thanks to the durable anchorage, the full range of flexural behavior was investigated, including post-debonding, comparison of strengthening using CFRP and GFRP, size effect in flexure of prestressed concrete beams and difference between the flexure capacities of beams which strengthened precracked and other strengthened at cracked region. The results indicate that the beams strengthened using prestressed CFRP and GFRP sheets exhibited a higher first-cracking, steel-yielding, and experimental nominal moments as the level of prestressing force increased up to a certain point.