A 45 GHz low power millimeter-wave (MMW) low noise amplifier (LNA) using 40 nm CMOS process is presented in this paper. The LNA comprises three stages. The first stage adopts the active feedback structure for good input matching. The output matching employs an L-type matching network. Simulation results show that both S11 and S22 are less than −18 dB at 45 GHz. A cascode structure is adopted in the third stage for better reverse isolation. The LNA consumes 18.7 mW under a 1.1-V voltage supply, achieving a maximum S21 of 17.9 dB. The minimum NF is 5.7 dB at 42.5 GHz. The simulation results show that the proposed LNA is promising for the 45-GHz application.