Fine powders of GaOOH nanocrystals are synthesized via a facile hydrolysis process through the solution-solution interface reactions of anhydrous GaCl3 and distilled water followed by subsequent solvothermal treatment at mild conditions. Well-faceted alpha-GaOOH hexagonal prism-like nanorods are prepared through solvothermal treatment at 180 degrees C with CTAB as the morphology controlling surfactant. Ga2O3 nanocrystals are fabricated via the pyrolysis of a-GaOOH hexagonal prism-like nanostructures at temperatures above 410 degrees C. A peculiar back-transformation from beta-Ga2O3 to alpha-Ga2O3 has been observed to occur between about 557 degrees C and 719 degrees C, which is considered to be responsible for the coexistence of the two phases. The phase transformation mechanisms of Ga2O3 at elevated temperatures, as well as the possible transformation route, have been postulated from a thermodynamic point of view.