Abstract: A Ka-band active vector synthesis phase shifter implemented in 65 nm silicon based complementary metal oxide semiconductor process is presented. The circuit is composed of orthogonal coupler, single-ended to differential signal balun, variable gain amplifier and a signal synthesis network. Based on lumped LC equivalent model, the quadrature generator can achieve compact size and obtain high precision orthogonal signal. The variable gain amplifier consisting of digitally controlled cascode topology is employed to perform precise gain tuning and improve the isolation between input and output. Measured results prove that the phase shifter can cover full 360°with 5. 625°phase step and provide accurate phase tuning with root mean square (RMS) phase error<3°and RMS gain error<1 dB over 25 GHz~32 GHz. The circuit area is 800 μm×400 μm and the power consumption is 11 mW.