In this work, we investigate workfunction (WK) fluctuation of gate-all-around Si nanowire MOS devices by solving a sets of 2D Schrödinger-Poisson equations. We discuss characteristic fluctuation in view of randomly interactive quantum confinement with subbands and wavefunctions. The influences of metal-grain size and channel width on the random WK-induced characteristic fluctuation are studied; additionally, the random positions of metal grain are discussed. The WK of metal grain in the corner of square-shaped channel possesses greater impact on characteristic fluctuation because of enhanced corner effect. Devices with a large channel width and small nanosized metal grains suffer from relatively smaller percentage of fluctuation.