A perturbed piezoelectric waveguide is constructed from a reference waveguide, which is specified by the eigenvalues and eigenfunctions, by the coordinate transformation, or other methods. In the perturbed waveguide, coupling occurs between normal modes of the reference waveguide. Coupling phenomena are analyzed by transforming Maxwell's and Newton's equations into the variational expression of the phase constant of the normal mode and then by applying the Rayleigh‐Ritz method. As an example, the couplings between purely elastic waveguide modes are discussed; and then the two‐mode approximation is applied in the case when two modes are degenerate or nearly degenerate. Mode conversion phenomena are discussed and from the viewpoint of the network theory, the mode transfer matrix relating the complex amplitudes of the two modes along the waveguide is defined and developed.