In this paper‚by means of theoretical derivation and experimental verification‚the effect of the thickness of carbon fiber laminate （CFL） on the failure mode of the strengthened RC （Reinforced Concrete） beam was investigated．In the investigation‚a method and corresponding equations were proposed to estimate the fa-i lure modes of4 groups of beams．The theoretically estimated results show that the failure mode of the RC beam strengthened with CFL changes with the thickness of CFL and that 4kinds of failure modes‚namely‚the tension fracture of CFL‚the debonding fracture of interface or the cover concrete separation‚the concrete crush and the mixed failure mode‚are respectively related to the4thickness regions．All these estimated results were finally verified by some tests．