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The size- and orientation-dependent uniaxial strain effects on ballistic hole transport in nanowire field-effect transistors are investigated using an sp3d5s*-based tight-binding formalism coupled with a compact electrostatics model and a semiclassical transport model. It is found that the strain-induced reduction of the valence band density of states leads to an increased ballistic hole current. This is explained by the product of a small reduction in hole density and a significant increase in the average ballistic hole velocity under uniaxial compression. While uniaxial compressive strain is beneficial for both 〈110〉 and 〈100〉 devices, the strain response of 〈110〉 nanowires is much larger than their 〈100〉 counterparts. Ultrascaled 〈110〉 nanowires have the highest hole drive current under both strained and unstrained conditions, despite the reduction of strain-induced ballistic hole current enhancement for narrower devices.