This paper concerns a performance-reliability based criterion for the optimum design of bridges isolators. The meaning of “optimum design” concerns a structure designed in order to satisfy several performance requirements regarding the safety and the serviceability. In order to carry out this procedure a stochastic approach is developed and a Gaussian, zero mean, filtered, non stationary stochastic process, is adopted in order to model the earthquake motion. The hysteretic Bouc-Wen model is employed in order to reproduce the non linear constitutive behaviour of isolators, whereas a linear law is assumed in order to represent the piers. Covariance response is attained by means of the approximate stochastic linearization method and the system reliability, required in order to make explicit the safety and the serviceability performance objectives, is evaluated in the hypothesis of independent crossings. Finally, the optimum design, performed in two different phases, is carried out in a parametric form.