As the technology node extends to sub-16 nm, the task of device scaling is getting more troublesome. Planar MOSFET could not satisfy the demand of performance. Hence, new structure such as multi-gate structure are proposed. Gate-all-around nanowire (GAA NW) MOSFET device is one of the most attractive structures. Besides, variability problems are becoming so crucial that they degrade the devices’ characteristic as well. Process variation effect (PVE), random dopant fluctuation (RDF) and work function fluctuation (WKF) are major fluctuation sources of all. In this thesis, exploring the influence of intrinsic characteristic fluctuation on GAA NW MOSFET with 10-nm-gate-length is conducted by full quantum mechanics theory and experimentally calibrated 3D quantum corrected device simulation. In the part of RDF, dopants penetrating from source/drain extension forms another unignorable fluctuation, which becomes more significant as the channel doping concentration decreases, so it must be included. The results show that the largest deviation of Vth is occurred as dopants are flocked at the place corresponding to the peak of conduction band profile regardless of RDF mechanism. In the part of PVE, the variations of gate length, radius, aspect ratio (AR) and oxide thickness are investigated. The results reveal that the fluctuation of oxide thickness is so small that it can be neglected, and the radius as well as aspect ratio (AR) have influential impact among all PVE factors. They have dependence on each other through a transformation formula. Although the device with large AR has poor SCE parameters, it has better suppression on PVE. Then RDF and PVE are combined to discuss the total impact. Small AR device has minimal fluctuation due to its own small effective radius, resulting in better channel controllability. In the part of WKF, the fluctuation is induced by the number and the location of metal grain with high work-function. The more metal grain with high work-function gathers near source side, the worse device’s characteristic is. Fabricating the size of metal grain which is far less than gate area can effectively reduce the characteristic fluctuation on device. Compared to tri-gate MOSFET, GAA NW MOSFET has great immunity to variation problems. As WKF and PVE are considered simultaneously, the device with small AR suffers more serious fluctuation, which is different from the trend of RDF. In summary, this thesis has discussed the impact of most important fluctuation sources on GAA NW MOSFET device. It provides semiconductor industry to develop some advanced device structures, materials and process technologies to promote the performance of chips.