Ruthenium clusters trapped in the supercage of NaY zeolite were prepared by the reduction of Ru under flowing H-2 at 623 K after a ruthenium-red complex was ion-exchanged into the supercage and subsequently decomposed under vacuum at 673 K. The cluster size was controlled by the heating rate used for the thermal decomposition, and the number of Ru atoms per cluster was estimated using a xenon adsorption technique. Various Ru/NaY samples in which the number of Ru atoms per cluster ranged from 20 to 60 have been used to investigate the effects of a decrease in Ru cluster size on the catalytic reaction rate of ethane hydrogenolysis. The reaction rates per surface Ru atom of a Ru/NaY cluster as compared with the bulk metal surface were approximately two orders of magnitude smaller, and the X-ray absorption near edge structure (XANES) taken at the Ru K edge (1s --> 4d transition) showed an energy shift. However, in spite of cluster size variation within the supercage, the reaction rates per surface Ru atom and the XANES did not change significantly. The activation energy for the reaction was similar for all Ru samples. These results indicate no significant electronic effects on catalytic activity as the cluster size decreased.