We investigate the impact of structure relaxation on the upper performance of a silicon nanowire metal-oxide-semiconductor field-effect-transistor (MOSFET) with small diameter employing a semiclassical transport model to calculate its ballistic I-V characteristics. For wires along the lang110rang axis and 1 nm diameter, structure relaxation induces large changes in the bond length of silicon atoms at the surface (~33%). Despite these bond length variations, the effect of reconstruction on the ballistic ON-current of Si-NW MOSFETs turns out negligible in the case considered, which can be attributed to an only slight variation of the electron effective mass after reconstruction