We report stable organic field-effect transistors (OFETs) based on pentacene. It was found that device stability strongly depends on the dielectric layer. Pentacene thin-film transistors based on the bare or polystyrene-modified SiO₂ gate dielectrics exhibit excellent electrical stabilities. In contrast, the devices with the octadecyltrichlorosilane (OTS)-treated SiO₂ dielectric layer showed the worst stabilities. The effects of the different dielectrics on the device stabilities were investigated. We found that the surface energy of the gate dielectric plays a crucial role in determining the stability of the pentacene thin film, device performance and degradation of electrical properties. Pentacene aggregation, phase transfer and film morphology are also important factors that influence the device stability of pentacene devices. As a result of the surface energy mismatch between the dielectric layer and organic semiconductor, the electronic performance was degraded. Moreover, when pentacene was deposited on the OTS-treated SiO₂ dielectric layer with very low surface energy, pentacene aggregation occurred and resulted in a dramatic decrease of device performance. These results demonstrated that the stable OFETs could be obtained by using pentacene as a semiconductor layer.