Uniform ruthenium metal nanoparticles with an average diameter of 5 mm were prepared and supported on gamma -Al2O3 UP to 6.3 wt%. The ruthenium nanoparticles were obtained by the reduction of RuCl3 in ethylene glycol. While maintaining their original morphology, more than 99% of the colloidal Ru nanoparticles, obtained were successfully supported on gamma -Al2O3. The Ru/Al2O3 catalyst prepared from a metal colloid had an unusually high activity for ammonia synthesis. The rate of ammonia formation was at least 12 times higher than the rate of nonpromoted Ru/Al2O3 catalysts prepared by conventional methods, and at least 3.5 times higher than the promoted Cs, Rb, K, Ce, La, and Sm Ru/Al2O3 catalysts. Alumina was not expected to be an ideal support for ammonia synthesis because of its acidity, and thus the promotion by additives was considered necessary. The present investigation proves that alumina can be a suitable support for ammonia synthesis if ruthenium is supported on it as metal nanoparticles. The results point out the possibility that the active sites formed by conventional impregnation methods are significantly contaminated with the supports. In contrast, by keeping the interaction with the support at a minimum, the novel preparation method of the Ru/Al2O3 catalyst by Ru colloid deposition gives well-defined metal particles with high catalytic activity.