The influence of particle size of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane (NH₃BH₃) was investigated. A silica-alumina composite shell was coated onto polystyrene (PS) template particles, before the removal of PS template particle by calcination. The diameter of the PS template particles decreased with an increase in the amount of poly (vinyl pyrrolidone) and the centrifugation speed, and the diameter of the obtained PS template particles was controlled between 150 and 400 nm. Hollow silica-alumina composite spheres with diameters of 180, 230, and 430 nm were prepared using these PS template particles; the shell thickness was controlled 15 nm by adjusting the amount of PS suspension and coating time. The hydrolytic dehydrogenation of NH₃BH₃ evolved 9.5, 9.0, and 7.0 mL hydrogen in the presence of hollow spheres with diameters of 180, 230, and 430 nm, respectively; the molar ratios of the generated hydrogen to the initial NH3BH3 in the presence of the hollow spheres were 2.5, 2.3, and 1.8, respectively. The results indicate that the activity increases with a decrease in the diameter of hollow spheres.