Abstract: Based on the traditional TE01n cylindrical cavity, a new resonator is proposed for non-destructive measurements of the complex permittivity of dielectric slabs. It consists of a large planar metal plate, a dielectric slab, and a cylindrical waveguide armed with a flange at one end and a tunable piston at another end. The dielectric slab is sandwiched between the metal plate and the flange. The rigorous mode matching method is employed for analyzing the electric fields in the cavity. Expressions relating the complex permittivity with the cavitys physical dimensions and the measured S21 parameter are developed. Thereafter, several dielectric slabs are tested to demonstrate the usefulness of this proposed method. It is found that by using this cavity, the measurement uncertainty for the relative permittivity is within 1%, while the measurement uncertainty for the dielectric loss could not exceed 10%. This cavity has the advantage of measuring the complex permittivity at multiple resonant modes. Therefore, it could be used as a robust approach for characterizing the frequency dependence of the complex permittivity of dielectric slabs.