Scanning tunneling microscopy (STM) has been employed to investigate the photoisomerization of azobis(benzo- 15-crown-5) on Au(111). A self-assembled monolayer of azobis-(benzo-15-crown-5) with a (3 x 7) symmetry was observed on Au(111). After ex-situ UV-light irradiation, a new adlayer structure with a different molecular arrangement could be seen on An(111), although the domain size of the ordered packing is small. On the basis of a high-resolution STM image and photochemical reaction result, schematic models have been proposed for the original and ex-situ irradiated adlayers. After in-situ UV-light irradiation, the ordered arrays disappeared and many bright clusters emerged at domain boundaries. The results presented here provide direct evidence at the molecular level for a photochemical reaction.