A wideband and flat gain distributed power amplifier using a GaN high electron mobility (HEMT) transistor has been designed. The frequency range covers 700 MHz to 4.5 GHz. The small signal gain has the average value of 10 dB. A reactive distributed shunt feedback structure is introduced and implemented by means of microstrip coupled lines. Also, fully distributed input and output impedance matching networks are implemented. The design and simulations are accomplished by advanced design system tool (ADS). The design has undergone large signal, small signal and electromagnetic analysis (EM simulation). At VDS = 28 V and IDS = 340 mA. Down to the output power back-off of 5 dB at 4.5 GHz, power performance obtained with PAE higher than 35% where the maximum output power is 40.4 dBm.