Nanowire transistors with a perfect crystal structure and a well-defined Si–SiO2 interface cannot be grown with the actual technology. The shape of the semiconducting channel varies from source to drain. By self-consistently coupling the three-dimensional Schrödinger and Poisson equations, interface roughness (IR) effects are studied in Si triple-gate nanowire transistors with , , , and  oriented channels. The full-band electronic transport is computed in the nearest-neighbor sp3d5s* tight-binding model. IR is included by adding or removing atoms at the Si surface. A comparison of the different channel orientations is achieved by calculating the variations of the transistor threshold voltage.