This thesis presents a way to make nonlinear device measurements for a power MESFET device using the load pull system. The device was characterized at the fundamental and second harmonic frequencies during large signal operation. The data thus collected was used in designing the input and output impedance matching networks that would optimize the performance of the device. A power MESFET device like the one used to conduct this experiment is mainly used in designing power amplifiers for communication systems including the transmitters used in satellites. Therefore efficiency of the part is of the utmost importance. By characterizing the device and utilizing matching impedance networks on the input and the output of the device, the efficiency of the device can be greatly improved. The characterization of the device, the construction of the matching networks, simulation and test results for the output power are all presented in this thesis.