Silicon nanowires (SiNWs) have recently been shown to be promising as high capacity lithium battery anodes. SiNWs can be grown with their long axis along several different crystallographic directions. Due to distinct atomic configuration and electronic structure of SiNWs with different axial orientations, their lithium insertion behavior could be different. This paper focuses on the characteristics of single Li defects, including binding energy, diffusion barriers, and dependence on uniaxial strain in , , , and  SiNWs. Our systematic ab initio study suggests that the Si-Li interaction is weaker when the Si-Li bond direction is aligned close to the SiNW long axis. This results in the  and  SiNWs having the highest and lowest Li binding energy, respectively, and it makes the diffusion barrier along the SiNW axis lower than other pathways. Under external strain, it was found that  and  SiNWs are the most and least sensitive, respectively. For diffusion along the axial direction, the barrier increases (decreases) under tension (compression). This feature results in a considerable difference in the magnitude of the energy barrier along different diffusion pathways. © 2011 American Chemical Society.