Nanowire transistors (NW Tr.) are promising device structures for ultralow-power LSI [1–3]. In our previous work, we demonstrated Ion improvement of short-channel NW nFETs and pFETs by adopting <100>-oriented NW channel instead of conventional <110> channel [3]. Although low-field mobility in <100>- and <110>-oriented NW Tr. have been thoroughly studied [3,4], the origin of higher Ion in short-channel <100>-oriented NW Tr. than in <110> NW Tr. is still unclear. In order to clarify the determining factors of Ion in short-channel devices, understanding of high-field carrier velocity as well as low-field mobility is required.