We investigate the performance of Ge and Si channel p-type nanowire Schottky barrier metal-oxide-semiconductor field-effect transistors (SB-pMOSFETs) based on rigorous quantum mechanical calculations. The multiband k . p method and the nonequilibrium Green's function are used. We find that Ge SB-pMOSFETs show superior performance in terms of ON-state current (I-ON), subthreshold swing, and the equivalent oxide thickness scaling. In particular, I-ON of Ge SB-pMOSFETs is estimated to become about 2.5 times larger than that of Si SB-pMOSFET if the possibility of achieving low Schottky barrier height (SBH) in Ge-channel devices is taken into account. As the channel width is scaled down to a few nanometers, however, the differences in device performance become smaller. This is explained by the increase of the tunneling effective mass due to the heavy-light hole coupling effect and the effectively increased SBH due to the size quantization effect.