Quantum simulations of multiple-gate nanowire Schottky-barrier (SB) MOSFETs in the ballistic transport regime have been performed by self-consistently solving the nonequilibrium Green's function transport equation and the Poisson's equation. The device characteristics have been examined as the channel length of the nanowire SB-MOSFETs was aggressively reduced, and their scaling behaviors were compared to planar SB devices and also to devices with doped source/drain. The enhancement of the device performance due to the multiple-gate effects has been assessed quantitatively. A limited improvement of the OFF-state performance has been observed, whereas ON-state currents increase significantly despite the size quantization effect.