The electronic structures of SnO2 doped by Al, Ga and In were examined using density functional theory. Density of states (DOSs) calculation showed that the Fermi levels of all Ⅲ-family doped SnO2 systems shift to lower energy and therefore a partially occupied valence band was formed near the valence maxima, which indicates that substitional Ⅲ-group atoms act as acceptors in SnO2. Partial DOSs indicates that compared with substitional Al, substitional Ga or substitional In contribute much more because of the d states, Ga3d or In4d, which suggests that Ga and In is better than Al for p-type doping of SnO2. Ionization energy calculation further indicates that,of the substitional In, Ga and Al, substitional In has the smallest ionization energy of 0.06 eV, which means that substitional In gives the shallowest acceptor level in SnO2, and thus the highest hole concentration at the same doping concentrations.