Abstract: The propagation characteristic of ultrasonic longitudinal guided waves in composite pipes is investigated. The dispersion characteristics were computed on the assumption that the frequency is a real number and the wavenumber is a complex number. The displacement amplitude component distributions were analyzed in a two-layer system. The optimal location, optimal mode and its frequency-thickness products of testing were chosen by the displacement amplitude component distributions. The results show that the values of the radial and axial displacements of guided waves are larger at the inner wall than those at the middle and outer walls of a two-layer composite pipe. When frequency-thickness products increase to a certain value, which will increase with the increasing of the order of mode, the radial and axial displacements approach zero. It should be the choice of frequency-thickness products where there are smaller radial displacement and larger axial displacement in testing of guided wave modes.