The paper deals with the seismic reliability of steel structural systems equipped with friction pen-dulum isolators (FPS). The behavior of these systems is analysed by employing a two-degree-of-freedom model accounting for the superstructure flexibility, whereas the FPS device behaviour is described by adopting a widespread model which considers the variation of the friction coefficient with the velocity. The uncertainty in the seismic inputs is taken into account by considering a set of artificial records, obtained through the power spectral density method as well as the friction coefficient at large velocity is considered as random variable modelled through a uniform prob-ability density function. Incremental dynamic analysis are developed in order to evaluate the fra-gility curves related to both superstructure and isolation level for different structural system prop-erties. Finally, considering the seismic hazard curve related to a site near L’Aquila (Italy), the seismic reliability of the overall system is evaluated providing reliability-based abacuses useful to design the radius in plan of the friction pendulum devices.