The orientation and size dependence on structural stability in Si nanowires (Si NWs) are investigated by using total-energy calculation based on a transferable-tight-binding (TB) model. The calculated nanowire (NW) cohesive energy for various shapes grown along < 110 >, < 112 >, and < 111 > directions with a diameter of similar to 4 nm shows that the hexagonal shape is preferable over the rectangle shapes. Furthermore, the cohesive energy in the hexagonal shape for diameter less than 6 nm demonstrates that the NWs oriented along the < 112 > direction is stable, consistent with experimental observation of Si NWs fabricated by the oxide-assisted growth (OAG). The stability of reconstructed surface in Si NWS is found to be interpreted as the sequence of surface energy, gamma{100} < gamma{111} < gamma{311} < gamma{110}, qualitatively consistent with experiments. These results imply that our approach provide firm theoretical framework to clarify the structural trends in Si NWs. (c) 2006 Elsevier B.V. All rights reserved.