This paper presents the results of an experimental research work designed to study the flexural strengthening capacity and the modes of failure of Reinforced Concrete (RC) rectangular beams which are strengthened with external bonding of carbon fiber reinforced polymer (CFRP) composite materials to the tensile face of the RC beams. In total, seven beams were cast, one beam was preserved as control beam, two beams were strengthened without the application of preloading and the four beams were precracked and repaired with CFRP sheets. The main experimental parameters include the original damaged degree of RC beams, the amount of CFRP sheets, the longitudinal tensile reinforcement ratio, shear span to effective depth ratio, and the concrete cover thickness. All beams were tested in four-point bending over a span of 1500mm. Test results in the current study indicate that the effectiveness and flexural capacity of the CFRP strengthened beams. The flexure enhancement of the CFRP strengthened beams varied between 41% and 125% over the control beam. This study confirms that the CFRP sheets technique significantly enhances the flexural capacity of reinforced concrete beams. Finally, the contribution of CFRP sheets on the flexural capacity and rigidity of precracked and non-precracked RC beams is significant for the more longitudinal tensile reinforcement ratio.