Vacancies are one of the most important defects in carbon nanotubes (CNTs). Vacancies could affect the mechanical, chemical, and electronic properties of CNTs. In this study, we first use first-principles plane-wave calculation to optimize the structure of single-walled CNTs with a double vacancy under 0%, 3%, and 6% strains, respectively. Then, we use the single-particle Green function method to calculate their transport properties. It is found that different strains cause different local structures near the defect, which change the transmission function around the Fermi energy, and the conductance tends to be maximized under ∼3% strain. © 2008 American Institute of Physics.