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A harmonic-tuned high efficiency oscillator is designed using gallium nitride (GaN) high electron mobility transistors (HEMTs). The harmonic load-pull simulation is performed to find the voltage and current waveforms and to locate the optimum load impedance for high efficiency operation of the transistor. Then, the feedback network for the oscillation is synthesized based on the load-pull data. The series resonant circuit is employed in the feedback network to provide open circuit to the load network at harmonic frequencies. Therefore, the load network can be designed separately from the feedback network to present the optimum harmonic load impedances. In this way, the transistor in the oscillator can achieve the optimum voltage and current waveforms determined by the harmonic load-pull simulation. The fabricated GaN oscillator using the proposed design approach shows the maximum efficiency of 80.2% and output power of 35.1 dBm at 2.42 GHz under drain bias voltage of 22 V.