Abstract: UV-Vis spectrum of ethyl 5-phenylazo-benzofuran-2-carboxylate in methanol indicates two absorption bands for fresh prepared solution. One is an absorption band of phenyl moiety, which is at 272 nm, the other is an absorption band of azo group, which is at 344 nm. Irradiated by light, a new absorption band at 460 nm appears for the solution. New compounds wasn't found in mass spectrum, thus, there occurs a cis/trans isomerization in azo group of ethyl 5-phenylazo-benzofuran-2-carboxylate in methanol. The cis/trans isomerization can reach an equilibrium, which is exposed in the room for 3 h. HPLC analysis shows the ratio of transisomer to cisisomer is 2.7, when the isomerization is in equilibrium.