Dielectric post resonances are investigated using a combination of the finite and boundary element methods (CFBEM). A substructure method is adopted in the finite-element method (FEM), so the dimension of the matrix equation to be solved can be reduced. Some of the lossless dielectric post resonances in a rectangular waveguide can be physically realised by a lattice circuit, and it is found that there are two types of dielectric post resonances. Moreover, the variations of the post location are related to the variations of the branch reactances of the lattice circuit.