Improvement in the performance of pentacene organic thin-film transistor has been demonstrated by using fluorine plasma-treated or ion-implanted HfO as its gate dielectric. The carrier mobility of the OTFT on HfO with 900-s plasma treatment can reach a carrier mobility of 0.662 cm2/Vs, about 7 times higher than that of the control sample without fluorine incorporation. The reason is larger pentacene grains due to trap passivation, smoother surface and higher surface energy. On the other hand, the carrier mobility of the OTFT on HfO with a fluorine implant dose of (1 ×10)14/cm2 is improved to 0.251 cm2/Vs, about 2.7 times higher than that of the control sample. The smoother surface of the gate dielectric with fluorine implant results in the growth of larger pentacene grains, leading to an increase of carrier mobility. However, excessive fluorine implant dose or plasma treatment time could cause damage to the gate dielectric, thus decreasing the carrier mobility of the device.