A novel non-orthogonal vector summing method is proposed and used to design phase shifter (PS) with low phase error and high phase resolution. The principle and topology of the proposed method are first investigated, while the calculation results indicate less sensitivity from phase error to magnitude mismatch than the conventional orthogonal vector summing method. A non-orthogonal vector generator with low phase and magnitude error is further designed to implement the proposed non-orthogonal vector summing. For demonstration, a $K$ -band full 360° PS with the phase resolution of 3.75° is implemented in a 65-nm CMOS process with the core chip size of 0.15 mm2. The results show a 0.75°–2.08° rms phase error and a 0.8–0.95-dB rms voltage gain error, while the total power consumption is 16 mW under a 1.2-V voltage supply.