A new method of obtaining a high‐density plasma (n∼nc0) in a resonant cavity is described. The method is to increase the frequency of the exciting microwave power source by as much as ∼ 10% while keeping the input microwave power constant and maintaining an impedance match between the source and the cavity. Density increases of the order of seven‐fold have been obtained in this way. The result is explained mainly on the basis that the power loss in the impedance‐matching network decreases as the excitation frequency is increased.