The localized electronic structure of single-walled carbon nanotubes (SWNTs) is qualitatively and quantitatively described by use of the first-principles calculations, and the localized molecular orbitals are distilled and the corresponding work functions are calculated within the framework of linear combination of atomic orbitals for molecular orbital. We find that the localized molecular orbital and localized work function are sensitive to the morphology of carbon nanotubes as like local density of states. With the localized electronic structures (i.e., localized work function, localized molecular orbital, and local density of states) of carbon nanotubes with various morphologies, we cannot only easily clarify the field emission characteristics of SWNTs, but also successfully interpret why open-ended SWNTs have higher chemical reactivity than capped ones.