A ferrite microwave harmonic generator is presented in the form of a rectangular cavity resonator. The ferrite-plate is attached to one wall and fills out the whole cross-section of the resonator which has a variable length. In the experiments a resonator length has been used at which the fundamental and the harmonic frequencies are resonant. Frequency doubling, tripling and quadrupling has been performed at input power levels up to the 10 kW range. Investigating various polycrystalline ferrites, best results were obtained with a Mg-ferrite at a thickness of 2.0 mm. Highest achieved conversion efficiencies were 40% with frequency doubling, 1.1% with tripling and 0.016% with quadrupling. A strong dependence of the efficiencies on ferrite thickness was observed.