Abstract: A new method to calculate the equivalent relative dielectric constant of the dielectric loaded ring-plane slow-wave structure is proposed. Expressions of dispersion equation and interaction impedance are derived for dielectric loaded ring-plane slow-wave structure by variational method. The results manifest that the dielectric-loading can lower the operating voltage of the ring-plane slow-wave structure. Good accordance was obtained between results by this method and that of CST-MWS.