In this paper, we propose an advanced nonlinear design technique for harmonic-tuned RF power oscillators, where the feedback, source, and load networks are independently designed based on the derived design equations. They present the transistor with the optimum impedances at fundamental and harmonic frequencies which are determined from the sourceand load-pull simulations. The proposed oscillator topology and design equations are verified by the simulations and measurements of RF oscillator using GaN high electron mobility transistors. The fabricated harmonic-tuned oscillator exhibits a maximum efficiency of 83% with an output power of 6.1 W at 2.45 GHz. To the authors’ knowledge, this corresponds to the best efficiency performance among previously reported RF power oscillators. The phase noise is also as low as −138 dBc/Hz at an offset frequency of 1 MHz.