The random nature of the explosion load, associated with the random nature of material properties, and geometric dimensional characteristics, implies the need to consider them into the reliability analysis in order to have a more correct estimation of the structural behavior. Therefore, when the randomness of these parameters in the analysis is considered, the response of the structure assumes probabilistic nature, and this makes it necessary to look into the reliability measure. This paper presents results from a parametric investigation of the reliability of reinforced concrete (RC) beam subjected to blast load. The probabilistic responses of the maximum displacement for a reinforced concrete flexural member under blast loadings are evaluated by means of nonlinear dynamic analysis with simplified equivalent single-degree-of-freedom (SDOF) system. Results of numerical simulations have shown the response of structures, in terms of maximum displacement in relation also to the blast load and the geometrical and mechanical characteristics of the beams. Monte Carlo simulation of dynamic response of the equivalent SDOF system is performed to estimate the reliability.