Abstract: This paper presents a harmonic-suppressed Class-J power amplifier (PA) operating at 1.8 GHz, implemented in a domestic 0.25-μm Gallium Arsenide Pseudomorphic High Electron Mobility Transistor (GaAs pHEMT) process. By employing an output matching network with high harmonic suppression capability, it achieves significant suppression of second-, third-, and fourth-harmonics, with measured distortion of -53.6 dBc, -66.0 dBc, and -82.6 dBc, respectively. The two-stage cascaded PA occupies a compact core area of 1.19 mm². Measurement results show a small-signal gain peak of 35.9 dB, input return loss better than -8 dB, and output return loss better than -5.8 dB. Across the 1.5 to 2.1 GHz band, the output power exceeds 26.5 dBm, with a peak power-added efficiency (PAE) of 56.5%. At the maximum output power point, the Amplitude Modulation to Phase Modulation (AM-PM) distortion remains as low as 4.5 degrees. This work effectively overcomes the limitations imposed by high-order harmonics on PAE and linearity, providing a high-efficiency Radio Frequency (RF) power amplifier solution with strong harmonic suppression for Sub-6 GHz communication systems.