Properties of the attenuated reflection and surface plasmon resonance (SPR) field distributions of both metallic and dielectric gratings placed on metal films were analyzed by using the rigorous coupled wave analysis. Relations between the gratings’ filling factors and the effective index(Neff) of the surface plasmon polaritons(SPPs) propagating on both structures were investigated. The results show that the increase of filling factor can only cause trivial changes to Neff of the SPPs on gold gratings, but distinct increments to that of the dielectric gratings.