A fully integrated 27-GHz transformer based power amplifier with neutralization technique and low-loss transformer is proposed and fabricated in 28-nm CMOS technology. Several common-source cells are combined together as differential power cells. On-chip transformers and current-combining topology are used to combine amplifiers and reduce the problem of output power loss. Using an aluminium-pad (AP) layer on output matching reduces loss and can keep high PAE. In order to improve the overall stability, a neutralization structure is utilized in the combined cell. The measurement results demonstrate a small-signal gain of 13.1 dB, saturated power (Psat) of 17.9 dBm, output power density of 513.8-mW/mm, and output 1-dB compression point (OP1dB) of 14.7 dBm at 27 GHz. The peak power-added efficiency (PAEpeak) achieved by this power amplifier (PA) at 27 GHz is 40.7%. The core size of the chip is 0.12 mm. To the best of authors' knowledge, this circuit presents a superior power and efficiency performance compared with reported Ka-band common-source CMOS PAs.