We report the first-principles calculation of the electronic distribution and the density of states (DOS) of the armchair(5,5) and zigzag (9,0) single-walled carbon nanotubes (SCNTs) grafted by carboxyl within the framework of density-functional-theory(DFT). It was found that the electronic structure of the SCNTs changes significantly due to the graft, namely, the electron density of states near Fermi level and the delocalized degree of the highly occupied molecular orbital decline greatly with increase of the number of carboxylic functional groups grafted. This suggests that the graft weakens the electronic transport properties of SCNTs.