In Brazil, the design of reinforced concrete structures follows the recommendations of Brazilian standard NBR 6118-2003. The Brazilian standard uses the methodology of semi-probabilistic design that does not quantify the probability of failure of a structural element. The reliability analysis of structures is a tool that allows the calculation of probability of failure associated with a limit state and also the design for a given probability of failure. This work aims to develop a methodology that enables the Reliability-based-design of concrete beams sections with and without strengthening with carbon fiber composites. In the reliability analysis were considered as random variables the loads, the compression concrete strenght, yield strenght, and tensile strength of carbon fiber composites with certain statistical properties based on the results of tests performed in the Structures and Materials laboratory at PUC-Rio (LEM-DEC). Examples of reinforced concrete beams strengthened with CFC were performed initially by semi-probabilistic methods and then using the proposed methodology based on reliability. The analysis shows the feasibility of a Reliability-baseddesign using a reliability index of failure probability determined for each type of project combining economy and safety.