First-principles simulation is used to investigate the structural and mechanical properties of vacancy defective single-walled (5,5) carbon nanotubes. The relations of the defect concentration, distribution and characteristic of defects to Youngs modulus of nanotubes are quantitatively studied. It is found that each dangling-bond structure (per supercell) decreases Youngs modulus of nanotube by 6.1% for symmetrical distribution cases. However the concentrative vacancy structure with saturated atoms has less influence on carbon nanotubes. It is suggested that the mechanical properties of carbon nanotubes depend strongly upon the structure and relative position of vacancies in a certain defect concentration.