The structures of mer-tris(8-hydroxyquinolinato)aluminum (Alq3) in the ground (S-0) and first excited (S-1) states have been optimized at the B3LYP/6-31+G(d) and CIS/6-31G(d) levels of theory. Absorption and emission for Alq3 are predicted with use of CI-ZINDO and TD-DFT. The results of an energy-partitioning analysis of the ground state of Alq3 are discussed. The electronic transition between S-0 and S-1 for mer-Alq3 is found to be mainly localized at the A-quinolate ligand as evidenced by the structural shift between the excited and ground states and the partial charge transfer from the phenoxide side to the pyridyl side within A-quinolate. The calculated optical and structural properties of mer-Alq3 are traced back to the weakest electrostatic attractive energy between the A-quinolate ligand and the Alq2 fragment due to the special arrangement, resulting in geometry change of ligand A upon excitation and the localization of the HOMO on A-quinolate.