A small sandwiched transition region between the Au catalysis droplet and silicon nanowires (SiNWs) is proposed to investigate the diameter-dependent orientation of SiNWs grown by the vapor-liquid-solid (VLS) mechanism. Atomic-scale calculation shows that for a given transition region width, there is always a critical diameter. Below the critical value, surface energy dominates and the 1 1 0 orientation is preferred, whereas at larger diameters, the interphase energy dominates and SiNWs grow along the 1 1 1 direction. The variability of the critical diameter is also included in our model by adjusting the transition region width. The theoretical results are in agreement with those from experiments. (C) 2008 Elsevier B. V. All rights reserved.