La2O3 and/or Ga2O3 doped, and the double dopant γ-Al2O3 composite powder samples were prepared by the sol-gel method. Nanostructural characterizations of the powder samples were performed by high resolution transmission electron microscopy (HRTEM) in order to clarify the role of the metal oxide dopants for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material. The results of HRTEM study and selected area electron diffraction (SAED) analysis revealed the formation of the uniform solid solution of Ga2O3-Al2O3. In addition, the results of X-ray elemental mapping analysis revealed the existence of well-dispersed La2O3 on the grain surface of Ga2O3-Al2O3 solid solution. These structural features could act an important role for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material.