Photo-isomerization of aromatic α-hydroxy hydrazone was reported. We investigated the structures of salicylaldehyde phenylhydrozone(SP) in the ground state using density functional theory(DFT) with the B3LYP functional and the 6-311+G(d) basis set. All nine possible isomers of SP in the ground state consist of seven phenol forms and two ketone forms. Intrisic reaction coordinate(IRC) analysis discloses the existence of a cycle driven by the two proton transfer processes in the ground and excited states of SP, which suggests that no ketonic form could exist in the ground state. Further theoretical studies of the potential energy surfaces support a trans-cis conversion followed by a relaxation to the stable form of SP in the excited states. We investigated the structures of salicylaldehyde phenylhydrozone (SP) in the ground state using density functional theory (DFT) with the B3LYP functional and the 6-311 + G (d) basis set. All nine possible isomers of SP in the ground state consist of seven phenol forms and two ketone forms. Intrisic reaction coordinate (IRC) analysis discloses the existence of a cycle driven by the two proton transfer processes in the ground and excited states of SP, which suggests that no ketonic form could exist in the ground state. Further theoretical studies of the potential energy surfaces support a trans-cis conversion followed by a relaxation to the stable form of SP in the excited states.