ZnGa2O4 fine particles with a single phase of spinel were synthesized from a mixed solution of gallium sulfate and zinc sulfate in the presence of aqueous ammonia under hydrothermal conditions above 180 degrees C, The effects of treatment temperature and ZnO/Ga2O3 molar ratio in the starting solution on the crystallite size, morphology, lattice parameter, and chemical composition of the ZnGa2O4 spinel particles were examined. Spinel with different morphologies, cubic nanoparticles, and elongated rodlike particles were thought to be formed based on the structure of amorphous gallium hydroxide and needlelike GaO(OH) particles, respectively. By treatment at a higher temperature, these particles with nonstoichiometric composition grew large and thick, and their composition approached ZnO/Ga2O3 = 1. With an increase in the starting ZnO/Ga2O3 molar ratio, the lattice parameter of the synthesized ZnGa2O4 spinel approached the reported value for the stoichiometric composition and reached a = 0.8335 nm at ZnO/Ga2O3 = 1.95 by treatment at 240 degrees C for 50 h.