The geometries and the harmonic vibrational frequencies for several AlSO doubler state isomers have been predicted at density functional theory (DFT) levels with a 6-311+G* basis set. Results have indicated that there are five stable doublet states (two cyclic, two linear and one bent structure) and only four of them keep S-O bond linkage, but the most stable one has a broken O-S linear structure and should be referred to as the ground-state ((2)Sigma) isomer. The state-state energy separations are determined and the properties of these low-lying state isomers are also characterized. The state-state correlation and the isomerization mechanism are predicted by searching the transition states.