The size of silicon grain in silicon thin-film transistors (TFTs) and the dependence of the gate-leakage current have been theoretically investigated after the effect of the silicon-grain size on the surface potential is considered. After the crystal-size effect has been included, the gate-leakage current of nanocrystalline silicon TFTs strongly depends on the silicon-grain size when the silicon-grain size is in the regime of nanoscale. Such a strong dependent relation results from the large changes in the band gap and dielectric constant due to size effects. The numerical calculations also demonstrate that the effect of the silicon-grain size on the gate-leakage current is independent (or weakly dependent) on the device temperature, the gate voltage, and the active-dopant density. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3372328]