Abstarct: In this research, Si<100>, <110> and <111> nanowires with different lengths between 0.5 to 20 nm and thicknesses between 1 to 4 nm, are investigated by using structural mechanics approach and numerical method by ABAQUS software. DREIDING is used for force field. Mechanical properties such as Young's module, shear modulus, Poisson's ratio are calculated. B.C is fix-free. The results of this study show that Young's module and shear modulus for all Si nanowires tendency toward Silicon bulk elastic properties in 7-10 nm thickness. Three structures of silicon nanowire such as <100>, <110> and <111> are modeled that <110> and <100> had the maximum and minimum Young’s modulus, respectively, howover maximum shear modulus and minimum is for <100> and <110> respectively. Analysis results show that bundle nanowires in a certain range of thickness of nano-wires is generally in the range of 4-3 nm, have a different behavior than the single mode. For example, the buckling analysis, Increase the critical load can be seen. Moreover, this method in comparison of other than methods such as molecular dynamics has higher analyzing speed and suitable accuracy.