For dielectric waveguides of arbitrary profile with step-index and gradient-index, the universal expressions of eigen mode expansions of dyadic Green s functions for dielectric waveguides have been found by using the approach of operator theory. It is shown that the expression of electric-type dyadic Green s function is the sum of an eigen-mode expansion containing nine dyadic components and a singular term. From these expressions the dyadic Green s function for a dielectric waveguide can be obtained if the eigen-mode of the waveguide are given.