The reflectivity of the surface polaritons incident normally on a grating of finite length is investigated without assuming that the amplitude of the grating is small. The maximum reflectivity is found to be essentially the same as that obtained from a small‐amplitude grating theory, but there is an upward shift in the center frequency of the band gap, with the shift increasing with the increase of the amplitude of the grating. A physical explanation for the shift of the band gap is provided.