Improvement　in　the　performance　of　pentacene　organic　thin-film　transistor　has　been　demonstrated　by　using　fluorine　plasma-treated　or　ion-implanted　HfO2　as　its　gate　dielectric.　The　carrier　mobility　of　the　OTFT　on　HfO2　with　900-s　plasma　treatment　can　reach　a　carrier　mobility　of　0.662　cm(2)/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　HfO2　with　a　fluorine　implant　dose　of　1x10(14)/cm(2)　is　improved　to　0.251　cm(2)/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.