Supercritical fluid (SCF) technology was proposed to improve the dielectric properties of electron-gun evaporated aluminum oxide (AlOx) film in this work. The leakage current of AlOx film deposited at room temperature was suppressed significantly from 10(-4) to 10(-10) A at a bias voltage of -20 V after the SCF treatment mixed with water and propyl alcohol. The evolution of the leakage conduction mechanism was confirmed theoretically from trap-assisted quantum tunneling to the Schottky emission process due to the reduction of electric traps in the AlOx dielectric film. In addition, our work demonstrated the application of SCF-treated AlOx gate dielectric to a pentacene-based thin-film transistor.