Fiber-reinforced elastomeric isolators (FREIs) are a new generation of seismic isolation devices in which steel plates are replaced by fibers. FREIs have a number of advantages over SREIs including; superior energy dissipation capability, potentially low manufacturing cost, light weight, and the possibility of being produced in long rectangular strips or large sheets and being modified to the required size locally. The main purpose of this work was to investigate the effect of shear strain amplitude and loading cycles on the horizontal characteristics of fiber reinforced nanocomposite elastomeric isolators (FRNEIs) which were manufactured based on nanocomposite rubber compounds. The results of typical isolator horizontal test showed that by increasing the number of cycles of loading and shear strain, the values of effective horizontal stiffness and shear modulus of FRNEI decrease and the viscous damping increases. This stress softening is known as the "Mullins effect", which occurs in filled rubbers.