To increase typically low output drive currents from Si Nanowire field-effect transistors (FETs), we show a GaN based GAA Nanowire FET’s effectiveness. The theoretical study is focused on the three dimensional device designs, comparisons, random dopant fluctuation using IFM, and general variability issues including nanowire length, gate work function, and channel thickness are discussed. Performance of GaN GAA Nanowire is found to be increasing as Gate length is increased. Electrical characteristics of FETs including threshold voltage saturation, On/Off current ratio and sub threshold slope (SS) are analyzed. GaN GAA structure let to gate control ability improvement compared to Si based Nanowire in electrical performance. The GaN GAA Nanowire subthreshold slope is ~62mV/decade, which is close to the theoretical limit 60 mV/decade and leads to very high Ion/Ioff ratio of 1010-1011. The GaN GAA Nanowire is a very promising candidate for high-performance.