Abstract: There exist various types of defect structures in carbon nanotubes(CNTs),which impact significantly CNTs performances.In this work,the first-principles simulation is employed to study the effects of defects of different sites and structures on CNTs properties such as the work function and the density of states(DOS).The investigation shows that the work function of the capped CNTs is 4.89 eV and the band gap is 0.7 eV.For the open-top CNTs,the work function is 4.7 eV and the band gap is extremely small,close to 0 eV.Compared with the capped tube,the work function of the open-top CNTs is lower and the band gap at the Fermi level is smaller.The site and structure of the defect are important factors affecting the CNTs properties.When the sites of the defects of same size move downward from the CNTs top,the work functions decrease.Meanwhile,the defect affects the CNTs conductivity.The tube bundle composed of metallic CNTs possesses a small band gap at the Fermi level,while the defect on the wall could make the band gap disappear,exhibiting better conductivity.