In this paper, the coupling characteristics of a dielectric directional coupler consisting of two parallel rectangular waveguides are analysed. We make use of variatonal analaysis to obtain the coupling length. It is shown that the presented technique is much more accurate than the widely used Marcatili s method as well ag pertrubation method. By means of this method, coupling length can be easily calculated with high accuracy, and it can be extended to the analysis of other devices n integrated optics.