This letter presents a 28-38GHz low-noise-amplifier (LNA) monolithic microwave integrated circuit (MMIC) using 0.15µm GaAs pseudomorphic high electron mobility transistor (pHEMT) process. Inductive degeneration and shunt-series peaking techniques are employed to obtain simultaneous input and noise matching (SINM) and wideband flat gain. A novel wideband multistage noise matching technique based on a high-pass matching network is proposed to achieve low NF and further improve gain flatness over a wide frequency range. A four-stage LNA prototype is designed and fabricated based on the proposed technique. Measurement results show that the proposed LNA has a 1.6-dB minimum NF and an average gain of 23.7dB in the entire operating band. S11 and S22 are better than -10dB from 30 to 37GHz. The measured output 1-dB compression point (OP1dB) is higher than 14.8dBm from 28 to 38GHz. The chip is realized within 2.1*1.5mm2 and consumes 56mA current from a 5V supply.