Flexural load tests were carried out to study the mechanical properties of deteriorated reinforced concrete（RC） beams strengthened with one or two layers of carbon fiber reinforced polymer (CFRP), including the characteristics of failure mode, the ultimate strength and the stiffness. The influence of CFRP amount on the ultimate strength and stiffness were focused on. And then the effective utilization of CFRP（γ） was discussed. It is shown that three failure modes appear as the deterioration degree increases, namely, crushing of compressive concrete, the tensile fracture of CFRP and shear fracture of the bonding layer. Before cracked, the amount of CFRP has small influence on the ultimate strength, and then the influence increases as the amount of CFRP increases when the beam cracked until the compressive concrete crushed first. The product of elastic modulus and sectional area of corroded rebar to elastic modulus of CFRP ratio determines the rigidity of strengthened beams. The effective utilization of CFRP γ can be determined by the formula Acf,min/Acf, max≤γ≤1, where Acf, min and Acf, max are the amount of CFRP sheets used corresponding to tensile fracture and shear fracture of it.