This paper presents a differential voltage controlled oscillator (VCO) implemented in a 65nm bulk CMOS technology for the unlicensed 60GHz band. It achieves a record efficiency of 4.9% by generating a maximum differential output power of -0.9dBm while drawing 16.5mA from a 1V supply (including buffers). This VCO is thus perfectly suited for the use in a fully integrated 60GHz transceiver for battery-powered wireless sensor networks (WSN). The minimum measured phase noise is -90.3 dBc/Hz at 1MHz offset and -112 dBc/Hz at 10MHz offset, the measured tuning range reaches from 57.6 GHz to 60.8 GHz.