A mobility model for organic thin-film transistors (OTFTs) has been considered that fully accounts for the effect of grains and grain boundaries of the organic layer. The model has been applied to a top contact pentacene OTFT. Comparison between simulation results and experimental data shows a strong dependence of mobility as a function of grain size. The field-effect-extracted mobility is not linearly related to the grain size, but presents a rather abrupt reduction for a grain size smaller than 2 μm