Intra mode coupling effects and degeneracy conditions are thoroughly investigated in the scenario of multilayer waveguide structures with grating. Propagating modes in uniform slab waveguides are exactly solved by a transfer matrix method, with modes’ profiles and dispersion characteristics illustrated. Modes coupling phenomena in parallel slab waveguides is studied through inspecting the spatial evolution of single mode’s profile as well as superimposed field distribution. Floquet-Bloch analysis is implemented to accurately solve the modes in single slab waveguides with periodic grating. Dispersion relations are obtained for both propagating modes and evanescent modes in the structure, where second order mode degeneracy is demonstrated at the regular photonic band edge. Through examining the fields’ distribution and spatial evolution, the fundamental harmonic and the -1st Floquet harmonic are illustrated. Finally coupled mode theory is implemented to support better understandings of the various coupling phenomena. Moreover, a coupled mode model is developed for the parallel slab waveguides with a grating structure in between, and the existence of the degenerate band edge in such stricture is qualitatively demonstrated.