Based on the anharmonic Keating model and the modified bending theory,an analytical semi-continuum atomistic lattice model is proposed for the size-dependent bending mechanical analysis of silicon nanowires (SiNWs) with different cross-sectional shapes.The semi-continuum model proves that the elastic behavior of SiNWs is sizedependent and predicts softer or stiffer SiNWs which depend on the relaxation coefficient and the surface tension.It also shows that the size effects of the mechanical responses of SiNWs depend on not only surface effects but also the cross-sectional shapes of SiNWs.Silicon nanowires (SiNWs) hold promise for a wide diversity of device applications,such as transistors,probes,resonators,sensors and actuators in nanoelectromechanical systems and biotechnology.[1-3] To achieve these applications,the exact characterization of the mechanical properties of SiNWs is a critical issue.Unlike their macroscopic counterparts with sizeindependent elastic properties,SiNWs exhibit a varying elastic modulus with their cross-sectional dimensions according to experimental measurements.[4-10]