This work presents a nonparabolicity (NP) model which is able to improve the effective mass approximation (EMA) for computing transfer characteristics of square silicon quantum wire transistors (SQWT) working in the ballistic regime and subjected to bandstructure effects. The model is found to be treatable within the same transport framework as used in a present 3D EMA Poisson-Schrodinger solver thus keeping a comparable time efficiency. A full-band tight-binding (TB) code provides the bandstructures as well as the transfer characteristics related to a series of SQWTs needed for calibrating the NP model. In comparison with the EMA, the threshold voltage (V-T) obtained via the NP model is notably closer to the TB data for all wire widths considered in this work. In addition, the NP model is found to satisfactorily predict the increase of the conduction masses belonging to the unprimed conduction valleys of the TB bandstructure. (C) 2009 Elsevier Ltd. All rights reserved.