In the fabrication of zinc oxide thin film transistor(ZnO TFT), aluminum oxide(Al2O3) was deposited as gate insulator by using an oxygen atom neutral beam assisted deposition(NBAD) during e-beam evaporation. From the thickness of the Al2O3 layer evaporated with the oxygen NBAD process and the C-f measurement of metal-insulator-metal(MIM) capacitors fabricated with the NBAD Al2O3 layer, it was possible to conclude that the NBAD Al2O3 layer has a higher thickness and a higher dielectric constant at an acceleration beam energy of 300 eV as a result of the additional deposition of oxygen atoms during e-beam evaporation. The fabricated ZnO TFT with the NBAD Al2O3 gate insulator showed better electrical characteristics, such as a lower subthreshold swing and a higher on-off ratio, than those without any NBAD process.