An improved finite element method for analyzing the eigenvalue problem of the open dielectric waveguides is presented. The basic idea of the method is an introduction of a transition region between the interior arid exterior regions. The introduction of the transition region in which the new adaptive interpolation functions are used

makes the total field approximation much more close to the practical field distributions than the simple classical treatment

and the quality of discretization much better. Dispersion characteristics for several dielectric waveguides are calculated by the improved finite element method. The comparison with the previous results obtained by other methods are made

and much better accuracy has been found. The reliability and the efficiency of the improved method are thus verified.