The filter characteristics of electromagnetic TM waves in a dielectric slab waveguide having a sinusoidal variation of its permittivity in the propagation direction are investigated. With the aid of a singular perturbation procedure, the coupled-mode equations governing the nature of TM waves interactions are deduced for both symmetric and asymmetric first-order Bragg interactions. Omitting the end effects, an expression describing the characteristics of reflection of TM wave is derived from the boundary value problem in the propagation direction. Experiments on reflection characteristics have been performed in the millimeter wave region where the dielectric slab created by arranging an Aflon slab in periodic fashion in a Teflon slab is tested as a permittivity-modulated slab waveguide. Return loss data and observed stopband responses are reasonably consistent with theory.