There is a growing interest in transistors based on Gallium Nitride in recent years upon development of new technologies. This master thesis presents the design of a wideband power amplifier operating in the bandwidth of 1.5 – 3 GHz based on a 10W GaN-HEMT from Cree. In this paper, relevant amplifier theory, detailed descriptions of design, fabrication and measurement processes are also presented. The amplifier design was done using a large signal model from Cree in Advanced Design System. Measurement results show good accordance with simulation results for the amplifier. Typical deviations from the ideal case are however evident and various factors contributing this result are discussed in this thesis. For better correlation of simulation and measurements, accuracy of passive component models and compensation of the voltage drop in power measurement are both considered in design process. In 1 dB compression point for the fabricated amplifier, 41.43 dB of output power, efficiency (PAE) of 68.27% is achieved. Furthermore, gain is measured to be above 10 dB over the bandwidth of 2.2 GHz in small-signal measurements