The photochemical reactions of stilbene and its derivatives have been extensively investigated in the gas and liquid phases but less on surfaces. In this work, scanning tunneling microscopy (STM) has been employed to investigate the photoisomerization and photodimerization of two stilbene derivatives on HOPG surface. After UV-light irradiation, one stilbene derivative performs photoinduced trans to cis isomerization and a well-ordered Kagomé network with two types of cavities forms at the 1-phenyloctane/HOPG interface, which is totally different from the lamella structures (before irradiation). Another stilbene derivative can also perform trans to cis photoreaction and the assembly structures have two conformations. With the increase of the irradiation time, one of the two conformations according to the topochemical postulates can further perform [2 + 2] photodimerization reaction. The difference in alkoxy substitution is reflected in the self-assembled monolayer and consequently also in the photoreactivity.