We report on the molecular weight effect of polystyrene (PS) gate dielectric on the characteristics of pentacene thin-film transistors. Dielectric layers were formed by using three different PS molecules of high molecular weight (PS-H) and low molecular weight (PS-L), and their blend (PS-B). The transistor having the PS-H gate dielectric exhibited the most pronounced drain currents as well as mobility. The gate-leakage current for the device with PS-H was even lower by one order of magnitude than that for the device with PS-L. The results are explained with the relationship between the surface characteristics of gate dielectric layer together with pentacene grain size and the transistor performances.