The discovery of the first carbon nanoutbe (Iijima, 1991)［1］has attracted wideattention and stimulated extensive studies. The studies show that the carbon nanotubesexhibit superior mechanical[2], electronic[3] and chemical[4] properties. On themechanical behavior, the carbon nanotubes possess exceptionally high strength,stiffness and elastic modulus. Their unique electronic transport properties give themthe potential to be the electronic devices of the future.[56] It would be expected that carbon nanotubes will bring a revolution to the nano science and technology。 A single-walled carbon nanotube (SWNT)[7］ can be viewed as a strip cut from aninfinite graphene sheet that is rolled up seamlessly to form a tube. The diameter and helicity of a SWNT are defined by the roll-up vector C=na1+ma2, whichconnects crystalline graphically equivalent sites on this sheet, a1 and a2 are thegraphene lattice vectors, and n and m are integers. Two particular classes ofsingle-walled nanotubes are worth mentioning. One is (n, 0), also called zigzagnanotubes, and the other is (n, n) or armchair nanotubes. Generic (n, m) nanotubes arecalled chiral nanotubes. In contrast, (n, 0) and (n, n) nanotubes are achiral(non-ehiral)。