A semi-continuum approach is developed to describe the effect of (2 × 1) surface reconstruction on the elastic modulus of the silicon nano-plate. Youngs moduli of a (001) silicon nano-plate along the high-symmetry [100]direction are obtained with and without considering (2 × 1) surface reconstruction. The approach predicts that the nano-plate with unreconstructed (001) surface is elastically softer than the bulk while it exhibits the opposite behaviour with (2 × 1) reconstructed surface. On the (001) surface, the (2 × 1) reconstructed surface dominates the plate as the thickness of the plate scaling decreases to several tens of nanometre. Whether the nano-plate is softer or stiffer depends on bond loss, bond saturation and direction of bond alignment, which have major impacts on the mechanics of the nano-plate.