Abstract: In this paper, a new method is presented for analysing rectangular-corrugated waveguides. First, with the concept of effective index of refraction, the rectangular-corrugated waveguide is transformed into a system of multilayered optical films and thus, the two-dimensional electromagnetic field problem is replaced by a one-dimensional problem instead. Then, by using the characteristic matrix method, we have obtained some important results of the rectangular-corrugated waveguide. The expression of coupled coefficient we obtained, is simpler and more precise in comparison with the results derived by conventional methods. Besides, the forbidden gaps of three kinds of rectangular-corrugated wavtguides have been calculated in detail and the derived centres of the forbidden gaps are actually distinct rather than coincident.