Co/Si core-shell nanowires (NWs) were fabricated by using highly conformai thermal atomic layer deposition (TH-ALD) of Co and the formation of Co suicide NWs by annealing was investigated. When Co/Si core-shell NWs were annealed without capping layer, no Co suicide was observed due to oxygen contamination. To prevent oxygen contamination, in situ TH-ALD Ru was used for capping layer utilizing its excellent conformality suitable for 3-D nanostructures as well as thermal stability. The scanning transmission electron spectroscopy analysis showed Si NWs are perfectly wrapped by TH-ALD Ru/Co films, resulting in the formation of Ru/Co/Si core-shell NWs. By annealing Ru/Co/Si core-shell NWs, CoSi2 was formed above Ta = 800 ° C. This silicidation by using TH-ALD Co and Ru can be effectively used for the fabrication of future nanoscale devices.