Thermal expansion behaviors of FeNi alloy layers in Fe 1 - x Ni x /Cu superlattices were investigated by means of X-ray diffraction technique in the temperature range of 10-300K. It was found that with the Invar composition (x=0.35), FeNi alloy layers showed low thermal expansions. But for alloy layers with either lower or higher Ni concentrations, thermal expansion coefficients were larger. It was inferred from results of magnetic measurements that FeNi alloy layers with Ni concentrations lower than the Invar composition were in a superparamagnetic state which could explain both a low spontaneous magnetization and a high thermal expansion rate of the alloy layers.