This letter presents the microwave performance of a sub-100 mu W Ku-band differential-mode resonant tunneling diode (RTD)-based voltage controlled oscillator (VCO) with an extremely low power consumption of 87 mu W using an InP-based RTD/HBT MMIC technology. In order to achieve the extremely low-power Ku-band RTD VCO, the device size of RTD is scaled down to 0.6 x 0.6 mu m(2). The obtained dc power consumption of 87 mu W is found to be only 1/18 of the conventional-type MMIC VCOs reported in the Ku-band. The fabricated RTD VCO has a phase noise of -100.3 dBc/Hz at 1 MHz offset frequency and a tuning range of 140 MHz with the figure-of-merit (FOM) of -194.3 dBc/Hz.