This paper overviews our current results on ISM-band high power and high efficiency GaN-HFET VCOs for use in the microwave intelligent heating. It describes mainly on the circuit design techniques using the imbalanced coupling resonator, the frequency variable resonator handling high power, the resonator-less feedback circuit, and injection-locked phase control. These design techniques can be considered as candidates for achieving higher power and higher efficiency of the oscillator type VCOs. With these techniques, miniaturized size and higher performance can be realized.