The hydrogenation of acetylene and ethylene was studied on Au/Al2O3 catalyst. Acetylene and hydrogen reacted readily to produce ethylene, but not to ethane on the catalyst in the temperature range between 313 and 523 K. The hydrogenation of ethylene on the catalyst occurred only at much higher temperatures over 573 K. The activity of the selective hydrogenation of acetylene over Au/Al2O3 catalyst depends on the size of ultrafine gold particles of Au/Al2O3 catalyst, which showed a maximum at about 3.0 nm in diameter.