A K-band variable-gain phase shifter using a Gilbert-cell vector synthesizer with a current digital-to-analog converter (DAC) and an RC-RL poly-phase filter (PPF) is presented, which is fabricated with a 65-nm RF CMOS process. It controls both gain and phase with low phase and gain variations as well as a low insertion loss. The Gilbert cell synthesizes a vector by summing in-phase (I) and quadrature (Q) phase signals of which the amplitudes are decided by the tail currents of the summing cells, respectively. The tail currents are given by current DACs, which are particularly designed to make the synthesizer have a constant output impedance in all phase and gain states. Because the gain and phase control resolutions depend on those of the current DACs, the size and insertion loss of the variable-gain phase shifter are not dependent on the control resolutions. The total root-mean-square (rms) phase and gain errors are measured to be 0.488 degrees and 0.098 dB for 4096 states of the 7-bit 360 degrees phase and 5-bit 17.8-dB gain controls, respectively, which are calibrated with extra 2-phase and 1-gain control bits. It shows -3.5 dB maximum gain including ON-chip balun losses with 6.6-mW dc power consumption.