Abstract: Dispersion curves provide the fundamental information to describe the propagation of elastic waves through a structure. The directional semi-analytical method combined analytical approach with numerical method is very efficient in solving complex problems. In this paper,dispersion relations for three-dimensional waveguides with rectangular cross-section are proposed by using directional semianalytical method.Resultsshow that a rectangular unit can be used to compute the dispersion curves for rectangular-rod. Geometric and physical parameters can affect the dispersion curves,and the influence on different modes is different. Finally,the proposed method is proven by the experiment.