Using Fourier analysis, expressions are derived for the power output and efficiency of a cavity-controlled Gunn-effect oscillator. This treatment predicts an optimum efficiency of 7.2% when the applied bias voltage is equal to 1.9 times the threshold voltage and the load resistance is equal to 8.6 times the low-field resistance of the GaAs wafer. The theoretical results are compared with the experimental resulu report by Dow, Mosher and Vane.