Field-plated (FP) AlGaN/GaN high electron mobility transistor (HEMT) has been fabricated and characterized for microwave applications. After the implement of the FP structure, lower transconductance (Gm) was observed on DC and small signal characteristics of the GaN power HEMT. Even FP-HEMT showed lower linear gain, the devices still exhibited higher output power and higher power added efficiency than conventional HEMT. However, lower trans-conductance and higher gate-drain capacitance (Cgd) didn't influence the linearity characteristics of the FP-HEMT. Finally, a 100μm GaN power HEMT on sapphire was fabricated with linear gain of 15.39dB, outpout power of 25.36dBm, and power added efficiency of 43%. The device also demonstrated high linearity of -27.1dBc for IMD3 and 33.88dBc for ACPR when biased at drain voltage of 30V with current density of 15mA/mm at 2GHz.