Silicon nano-wires(Si-NWs) are attractive candidates for electrodes in Li-ion batteries because they provide less constraint on the volume changes during Li charging. Recent experiments show that crystalline Si anodes expand highly anisotropically and can fracture from the surface during lithiation and not in the interior, as had been predicted by analyses based on diffusion-induced stresses. In the present study, we have tried to develop a model to explain experimental observations including surface fracture during lithiation and fracture due to anisotropic expansion. Compared to the diffusion-induced analysis, we modeled the sharp phase boundary between the crystalline silicon and the amorphous lithiated silicon, as well as anisotropic phase growth, by specifying an evolving concentration profile. Due to history dependent plastic deformation, crystalline Si-NWs fracture from the surface. In addition, we have tried to compute the critical size for fracture of Si-NWs with different crystallographic orientations using a cohesive zone model. By this analysis, our simple model may shed light on the mechanics of failure of crystalline Si-NWs during battery cycling.