The thermal stability of the parallel motif triple helix DNA containing CG.CH+ base triplets is improved by silver ions. The N3 protons of cytosines on the third strand are replaced by the ions to form CG.CAg+, which are stable regardless of the pH values. The activity of the DNAzyme that binds its substrate DNA through triple helix formation was examined in the presence of silver ions. The DNAzyme activity was retained even at neutral pH with the assistance of silver ions.