Externally bonded FRP may increase the fatigue life of RC beams due to the reduction of the steel stress level if compared to unstrengthened beams. In general, fatigue failure is marked by the gradual deterioration of the steel rebars, which leads a stress transfer to the Fiber Reinforced Polymer (FRP), until failure. Steel stresses higher than 80% of yield stress are associated with FRP debonding, while steel stresses below the fatigue limit will never lead to fatigue failure. The lack of data regarding fatigue life of FRP strengthened beams, mainly when related to low stress levels, leads to general fatigue analysis with S-N curves developed for different FRP systems and stress ranges. Aiming to improve the fatigue life analysis of RC beams strengthened with FRP this paper aims to develop specific S-N curves for different FRP systems, through the Maximum Likelihood Estimation Method (MLE). Experimental data with runouts of fatigue tests found in literature and generated through an experimental program were used to adjust the fatigue model aiming to characterize the large scatter between the results of fatigue tests from the different sources. The proposed fatigue models for the different FRP systems represent accurately the declivity of the fatigue curves for FRP strengthened RC beams.