Abstract This paper presents a standard program using finite-element method to calculate dispersion characteristics of hybrid dominant and higher order modes in rectangular waveguide filled with arbitrary block-shaped dielectrics. The dispersion characteristics calculated with the program for more than ten different rectangular waveguides loaded with various dielectric distributions verify the reliability of the program. The useful bandwidth of waveguide filled with I-shaped dielectric is analysed in detail. The dispersion curves given in this paper can be used in analysing and designing related microwave components.