This paper presents a 5-bit dual-band active phase shifter for both 28-GHz and 60-GHz phased array application. With the inclusion of a dual-band hybrid transformer-based quadrature generator and a wide-band vector modulator, this phase shifter achieves 3-dB bandwidth of 27-34 GHz and 5563 GHz. The vector modulator is composed of a balun-loaded buffer and a Gilbert cell-based variable gain amplifier (VGA). Further, a digital-to-analog convertor (DAC) controls the gain of Vector modulator to achieve 5-bit phase resolution. The circuit is designed in a 65-nm CMOS technology. Post layout simulation results show that the RMS phase error is 2.27◦ ∼ 2.94◦ and 1.96◦ ∼ 3.07◦ and the RMS gain error is 1.17 ∼ 1.53 dB and 1.26 ∼ 1.91 dB in 3-dB bandwidth of 27-34 GHz and 55-63 GHz respectively. The post-layout simulated input-reffered P1dB is larger than -8.7 dBm, while the peak gain is -0.17 dB at 29 GHz with an average power consumption of all phase states of 32.4 mW. The core area of the phase shifter is 560 µm × 560 µm. Index Terms—active phase shifters, CMOS integrated circuits,