The electrochemical deposition process of palladium from a PdCl42- complex on an Au(100) substrate was investigated using in situ scanning tunneling microscopy (STM). The reactant, i.e. the PdCl42- complex, was found to adsorb on the Au(100) surface with an ordered structure in the potential region where neither a cathodic nor an anodic current flowed. The electrochemical deposition of palladium on the Au(100) proceeded at potentials more negative than 1.0 V. The electrodeposition of the first palladium layer started not only on the terrace but also on gold islands which were formed as a result of the lifting process of the reconstructed Au(100) surface and then proceeded two-dimensionally. The following palladium layer was found to grow mainly from the deposited palladium region on top of the gold islands. The shape of the gold island could be identified even after four palladium layers were deposited. The bulk and surface structures of the deposited palladium layer were characterized by X-ray diffraction (XRD) and the copper underpotential deposition (upd) reaction, respectively, and the formation of the Pd(100) phase with a (1 x 1) surface structure was confirmed.