Thermal engineering assisted by electrical annealing was applied to enhance the device performance of a gate-all-around (GAA) silicon nanowire (Si-NW) transistor. The ON-state current is increased by four times. Joule heating was produced in a Si-NW by electrical biasing. The heating was concentrated on both edges of the gate, which served as a heat sink, effectively lowering the parasitic external resistance of the GAA Si-NW transistor. The electrical biasing gives rise to a thermal annealing effect on a selected device and to all devices connected by a common biasing electrode. The evidence reported in our previous work regarding current-induced oxidation by Joule heating in a Si-NW was also observed in the measured transfer characteristics of the GAA Si-NW transistor in this paper