Supported Pd catalysts have been modified with Si deposited on them by silane decomposition. When used in acetylene hydrogenation, the Si-modified catalysts show higher selectivity for ethylene and produce less amount of green oil than the unmodified Pd catalysts. LR and XPS analyses of the catalysts suggest that Si covers the Pd surface as Si or SiO2 patches. The Pd surface is diluted with the deposited Si as observed by IR spectra of CO adsorbed on the surface. However, the electronic property of the Pd surface seems to be unaffected by the Si species because such a modification is not observed in XPS nor acetylene-TPD analysis. Accordingly, we conclude that improved performance of the Si-modified catalysts comes mostly from geometric modification of the Pd surface by Si.