To investigate properties of SiNW-FETs, we have studied the band structures of SiNWs aligned to  direction with different diameters. Band structures of SiNWs have been derived by first principle calculation. In SiNWs aligned to  direction, effective masses of electron are much smaller than those of hole, while numbers of subbands (quantum channels) near the conduction and valance band edge don't have large difference. Those two factors determine the performance for a transistor. Also, inter-band scattering event is more frequent when intervals of subbands are narrower. Then, a trade-off model of the number of quantum channels has been proposed for an optimum diameter extraction.