Single atomic-vacancy related changes in the geometry and electronic structure of the single-walled carbon nanotube tip are revealed by the first-principles calculation. A distorted pentagon-octagon pair is formed in situ. The localized states, induced by the dangling bond, mainly dominate the edges of occupied and unoccupied states and fill the gap. The electron emission property of defected tubes becomes worse because the advantageous contribution from electronic effects is a bit smaller than the disadvantageous contribution from geometrical effects. Their electron emission behavior and pattern are mostly determined by the dangling bond at the octagon. (c) 2006 Elsevier B.V. All rights reserved.