Non-equilibrium Molecular Dynamics (NEMD) simulations of a stretched Lennard-Jones (LJ) model single crystal nanowire with square cross-section are carried out. The microstructural and mechanical properties are examined as a function of strain and strain rate. The instantaneous Poisson's ratio and Young's modulus are shown to be strongly time (strain) dependent from the start of the pulling process. The structural transformation as a result of straining initially involves the (100) layers moving further apart and then slipping at ca. math formula when the shear slip stress along that direction is about 1% of the shear modulus, which is typical of plastic deformation of noble gas solid crystals, and in accordance with Schmid's law.