In the current work, the contribution of gate oxide/channel interface roughness has been estimated and incorporated into the transport models of a Si nanowire field-effect-transistor (Si NWFET). This has been incorporated by modifying the relevant energy sub-bands taking into account the roughness in both transverse and longitudinal directions. Accordingly, the channel Hamiltonian matrix elements related to energy sub-bands are modified. It has been observed from the study that such interface roughness has significant effect on the device performance in terms of transfer and output characteristics. The transfer characteristics show that the current decreases up to 40% for an increase in interface roughness up to 25%. © 2012 IEEE.