Dispersion equation for waveguide modes of rectangular metal waveguide containing a dielectric buffer/graphene/air gap in the approximation of perfectly conducting waveguide metal walls, and the dispersion equation for waveguide modes of planar metal waveguide containing a dielectric metal/buffer/graphene/air were presented here. Dispersion dependences of TE0m modes for sub-terahertz frequencies versus Fermi level of graphene for different buffer layer thicknesses will plotted. Particular examples and modulator designs will be discussed for f= 0.3 THz. This paper highlights the unique advantage of using graphene together with dielectrics for sub-THz waves propagating in a planar and rectangular metal waveguide with a monolayer of graphene