Poly(amidoamine)(PAMAM) dendrimers with a cinnamoyl shell were prepared by reacting full generation PAMAM dendrimers (G=3.0) with 2-chloroethanol and cinnamoyl chloride, which resulted in densely packed polymerizable unsaturated groups on the periphery. The cinnamoyl shell of the dendrimers dimerized when irradiated under a UV light by using 5-nitroacenaphthylene as an initiator in dilute dimethylformamide (DMF). FTIR, H-1 NMR, UV-Vis, SEC, and a viscosity test certified that the photocycloaddition of the cinnamoyl shell of the dendrimers took place within the molecules with the disappearance of double bond signals in the FTIR. H-1 NMR spectra as well as the intrinsic viscosity and polydispersity value of the products both before and after irradiation showed no difference. It was further found that the cinnamoyl shell-modified dendrimers possessed fluorescence property, and the fluorescence intensity became stronger when the shell was photocyclized under UV-irradiation.