Abstract: Using the one-dimensional monatomic chain as a model, the vibrational properties of the carbon nanotube with pentagon and atomic vacancy have been investigated respectively. It explains qualitatively the reason that the two defects can induce the localized defective modes. From the calculations, it＇s found that the bond length on the pentagon rings is shorter than that of the hexagon rings, meaning that the force constants on the pentagon de- fect should be larger relative to those on the hexagon rings, A similar result is proved for the atomic vacancy. The effective mass of the vacancy is smaller than the normal carbon atom, at the same time, the bond length around the vacancy is shorter than the hexagon ring＇s. Therefore, the two kinds of defects may be considered as the ＇ light- mass impurity＇ , which can cause a new localized defect mode out of the vibrational modes of the perfect lattice, re- spectively.

Key words: one-dimensional monatomic chain, lattice vibration, topological defect, atomic vacancy, local- ized mode