Abstract: A technique has been demonstrated to significantly reduce the noise level in magnetrons. This method employsthe dispersion relation of the magnetron to find out the possible interaction parasitic modes, and find out the solutions. Noisemeasurements are carried out with the microwave oven in the standard EMC test laboratory. The related noise reduction at theparasitic mode is above 7dB and these greatly improved the EMC test pass rate from 50% to 80%.