The general expressions of the spectral-domain dyadic Green's functions for an inhomogeneous cylindrical waveguide and an inhomogeneous cylindrical cavity are formulated by the method of eigenfunction expansion and scattering superposition. The excitation current source can be in an arbitrary region. The dyadic Green's functions are used to calculate the phase constants and the cut-off characteristics of a dielectric-loaded cylindrical waveguide and the resonant frequencies of a dielectric-loaded cylindrical cavity. Complete agreement with published data is obtained.