This paper presents a highly linear cascode power amplifier (PA) for 5-GHz 802.11ac (wireless local area network) WLAN applications, which is fabricated with a 0.13-mu m standard RF CMOS process. A parallel-cascoded configuration is proposed to cancel out third and fifth intermodulation distortions and third harmonic distortion (HD) due to drain-source current nonlinearity. This also reduces distortions due to drain-source and gate-source nonlinear capacitances at both common source (CS) and common gate (CG) stages. The configuration allows the amplifier linear characteristics to be robust against gate node voltage variations of CG transistors compared to previous multigated transistor linearization methods, because the CG transistors always remain in the saturation region and the nonlinearities of capacitances associated with CG transistors cancel each other under a wide range of output powers. In addition, an active feedback linearizer is applied to improve AM-AM and the power-added efficiency (PAE) at high output powers. At 5.15 GHz, the proposed PA is tested with a 256-quadrature amplitude modulation WLAN 802.11ac signal source without digital predistortions. The output powers satisfying the stringent linearity, a -35-dB error vector magnitude, are 17.8, 17.3, and 15.6 dBm with 11.5%, 10.4%, and 7.5% PAEs at 20, 40, and 80 MHz, respectively.