Electrosorption of oxygen and CO on Ru(0001) surface were investigated by ex-situ electron diffraction (LEED/RHEED), AES and electrochemical data. The Ru(0001) surface exhibited a strongly affinity for oxygen which gave preference to form a (2×2)-O phase. However, anodization of Ru(0001) electrode produced a (1×1)-O phase. A coadsorption structure (2CO+O)(2×2) / Ru(0001) was performed from electrosorption of CO on Ru(0001) surface in electrolyte solution.