This paper investigates the feasibility of using a lanthanum oxide thin film (La_2O_3) with a high dielectric constant as a gate dielectric on GaAs pHEMTs to reduce gate leakage current and improve the gate to drain breakdown voltage relative to the conventional GaAs pHEMT. An E/D mode pHEMT in a single chip was realized by selecting the appropriate La_2O_3 thickness. The thin La_2O_3 film was characterized: its chemical composition and crystalline structure were determined by X-ray photoelectron spectroscopy and X-ray diffraction, respectively.La_2O_3 exhibited good thermal stability after post-deposition annealing at 200, 400 and 600 ℃ because of its high binding-energy (835.6 eV). Experimental results clearly demonstrated that the La_2O_3 thin film was thermally stable.The DC and RF characteristics of Pt/La_2O_3/Ti/Au gate and conventional Pt/Ti/Au gate pHEMTs were examined.The measurements indicated that the transistor with the Pt/La_2O_3/Ti/Au gate had a higher breakdown voltage and lower gate leakage current. Accordingly, the La_2O_3 thin film is a potential high-k material for use as a gate dielectric to improve electrical performance and the thermal effect in high-power applications.