The design of a wide-tuning-range millimeter-wave CMOS VCO is presented in this paper. In contrast to the conventional wideband topologies, a nonuniform standing-wave oscillator utilizing tapered gain elements, switched transmission lines and distributed varactors is employed to provide an extended output range with the coarse and fine frequency tuning. Due to the use of the transmission line architecture and the position-dependent amplitude of the standing waves, the loading effects of the varactors and the MOS switches can be alleviated, enabling the VCO to operate at higher frequencies. Using a 0.18-mum CMOS process, a 40-GHz VCO is designed and implemented. Consuming a DC power of 27 mW from a 1.5-V supply voltage, the fabricated circuit exhibits a frequency tuning range of 7.5 GHz with an output power level ranging from -13.6 to -4 dBm. The measured phase noise at 1-MHz offset is lower than -96 dBc/Hz within the entire frequency range. This work demonstrates the widest tuning range in percentage among the CMOS VCOs at millimeter-wave frequencies.