This paper presents the time-variant flexural reliability analysis of a RC highway bridge strengthened with CFRP. Three deterioration factors are dealt with. The first is the growth of traffic live load over time. The second is the resistance deterioration of steel reinforcing bars due to corrosion. The third is the deterioration due to aging of CFRP. The reliability analysis is performed implementing five flexural failure modes. A Monte-Carlo simulation is used to develop time-dependent statistical models for steel areas and daily live load extreme effects. The reliability of a RC bridge CFRP-strengthened interior beam, designed in accordance to AASHTO-LRFD (Load and Resistance Factor Design) specifications, is evaluated under various traffic volumes, different corrosion environments and strengthening configurations. The results of the analysis have shown that, among the deterioration factors, corrosion has the most significant effect on a bridge service life followed by live load growth. We introduce a probabilistic-based strengthening efficiency factor which allows comparing different types of CFRP or assessing influence of plate end anchorage options. The article ends with a discussion about the safety factor of flexural limit state showing the inefficiency of traditional design method which does not predict the accurate failure modes instead of probabilistic approach.