A novel water-soluble polyazobenzene dendrimer modified with L-lysines at the periphery was synthesized. The results from light scattering and gel filtration chromatography showed that the particle size is controllable upon UV or visible light irradiation. Furthermore, a gel shift assay with plasmid DNA and an electrophoretic light scattering analysis demonstrated that the affinity of this cationic dendrimer toward DNA is photo-controllable on the basis of zeta potential alteration on dendrimer's surface.