We have studied theoretically the electronic structure of a Pd monolayer on Au(111) and Pd(111) electrode surfaces by using the density functional theory. These electrode surfaces have been modeled by clusters consisting of 7 atoms on the first layer and 6 atoms on the second and third layer, and by slab models. We have adopted a CO molecule as an adsorbate. The properties of cluster models are calculated at the level of B3LYP, while the properties of slab models are calculated using a plane wave basis set and ultrasoft psuedopotentials. The calculated charge on the first layer of the Pd-Au clusters are more positive than that of the Pd-Pd cluster. As compared with the Pd-Pd cluster, electron transfer from the Pd-Au cluster to an adsorbed CO is restrained. The adsorption energy of the Pd-Au clusters are also restrained. We will show whether the effect of the bond length difference on the surfaces or the electronic effect of the second layer atom is stronger.