In the project of engineering structures, some design variables are usually taken as deterministic, although there are uncertainties associated with loads, material and geometrical properties. The use of safety factors is the most common strategy to deal with these uncertainties. The reliability analysis of structures is a tool to assess the probability of structural failure in a certain behavior and the sensitivity of this failure in relation to each variable considered. This work is concerned with the reliability analysis of the main beams of an existing bridge made of reinforced concrete, which is checked at the ultimate limit state in simple bending and the ultimate state of service in the formation of cracks as recommended by NBR6118: 2003. Matlab routines are developed in order to assess the probability of failure of the bridge using two methods: Monte Carlo simulation and FORM (First Order Reliability Method). Additionally, a sensitivity analysis is performed in order to analyze the influence of each variable in the reliability of the bridge.