Low-frequency vibritional modes are calculated for poly (dT). poly (dA).poly(dT) triple helix by the lattice dynamical theory. Long-range interaction is found tobe important for low-ftequency vibtitional mode. Both Watson-Crick hydrogen-bond andHoogensteen hydrogen-bond have a cluster of breathing modes centered about 90 cm-1,but Hoogensteen hydrogen-bond has another several very strons breathing modes in lowerfrequency. This result explains qualitatively the melting of poly(dT).poly(dA).poly(dT) in low salt solution.