This letter presents a 28-GHz CMOS linear power amplifier (PA) for 5G beamforming front-end that has dual power modes. The proposed PA consists of parallelly connected cascode cells, which are used to linearize the PA in the high-power mode (HPM). Power modes are implemented by splitting the cascode cells again. To have a low-power mode (LPM), half of the cells are turned off while halving the supply voltage simultaneously. This concurrent change makes it possible to keep the phase of the PA unchanged over two power modes while the dc power consumption is reduced by about half in the LPM. In addition, the good linearity is maintained in the LPM. The PA has gains of 18/14 dB, saturated output powers of 18.5/12.2 dBm, P1dBs of 16.5/10.2 dBm, and peak PAEs of 27.3%/17.8% with a 28-GHz continuous wave (CW) signal. The maximum linear output powers that satisfy EVM < -25 dB are 7.5/1.5 dBm with an 8-CC x 100-MHz 64-QAM OFDM signal.