A lattice dynamics model for poly(dT)·poly(dA)·poly(dT), which is one of the best important triplex DNA was presented based on its screw symmetry. In this model, the presence of Na + ions was included explicitly. The breathing modes of poly(dT)·poly(dA)·poly(dT) were calculated. By comparing the isolated triplex vibrational modes, it was found that the addition of Na + ions could extinguish very strong breathing modes of Hoogensteen hydrogen bond located in 18.4cm 1?36.4cm 1 and 43.8cm 1. The breathing modes of Watson Crick hydrogen bond were not changed obviously. These results indicate that Na + ions can increase the stability of poly(dT)·poly(dA)·poly(dT) and reasonably explain the melting experiment of poly(dT)·poly(dA)·poly(dT).