Rectangular waveguides with nonperiodical corrugation of wall are considered. The Krylov-BogoliubovMitropolsky asymptotic method is used for solving boundaryvalue problem describing electromagnetic wave propagation in the close waveguiding structure. Mutual transformations of a wave modes propagating in such waveguides are investigated. Analytical solutions for some coupling coefficients are obtained.Index Terms-Bragg mode conversion, corrugated waveguide, quasiperiodic structures, asymptotic method.