The conductive path formed by the interstitial Cu or oxygen vacancies in the Ta(2)O(5) atomic switch were investigated in detail by first-principles methods. The calculated results indicated that the defect state induced by the interstitial Cu is located just at the Fermi level of the Cu and Pt electrodes in the Cu/Ta(2)O(5)/Pt heterostructure and that a conduction channel is formed in the Ta(2)O(5) film via the interstitial Cu. On the other hand, oxygen vacancies in Ta(2)O(5) do not form such a conduction channel because of the lower energy positions of their defect states. The above results suggest that the conductive path could be formed by interstitial Cu in the Ta(2)O(5) atomic switch, whereas the oxygen vacancies do not contribute to the formation of the conductive path.