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This paper presents a CMOS vector-sum-based phase shifter with a flat group-delay deviation of 6.3 ps at K-band. The theoretical analyses confirm the effects of the nonideal phase-invertible variable attenuator (PIVA) on the increasing of the root-mean-square (rms) phase error and magnitude variation of the phase shifter. The design procedures, which can eliminate the effects from the voltage-dependent parasitics of the reflection load and the nonideal directional coupler, are reported to make an ideal phase reversal and reduce the attenuation-dependent phase shifting of the PIVA. The prototype of the phase shifter, which consists of one 3-dB directional coupler, two PIVAs, and one power combiner, are realized by using synthetic transmission lines for the circuit miniaturization. The circuit is fabricated in standard 0.18-μm 1P6M CMOS technology with a size of 560 μm × 560 μm excluding the testing pads. The comparisons between simulated and experimental results show that the prototype can cover full 360° phase shifting with less than 1.5° of rms phase error and 0.45 dB of magnitude variation from 22.0 to 26.0 GHz. The input P1dB and IIP3 of the prototype with a relative phase shifting of 292.5° are 12.0 and 28.0 dBm, respectively.