This paper presents a CMOS broadband millimeter wave power amplifier (PA) based on magnetically coupled resonator (MCR) matching network. The MCR matching network is analyzed theoretically. Design method for MCR-based broadband PA is proposed. For the PA's output matching network, the inductance ratio should be equal to the load/source resistance ratio to achieve broadband impedance transformation. And the coupling coefficient (k) of the MCR can be determined from the no gain ripple condition. Fabricated in 65-nm CMOS process, the PA chip achieves 32.9% peak power added efficiency, 15.3-dBm saturated output power (Psat), and 12.9-dBm output 1-dB compression point (P1 dB). The fractional bandwidth of the PA is 63.3% from 21.6 to 41.6 GHz, which covers the full Ka-band (26.5 to 40 GHz).