A series of hyperbranched glycogen derivatives conjugated with different contents of cationic 3-(dimethylamino)-1-propylamine residues, named DMAPA-Glyp derivatives, were synthesized at room temperature. Resonance light scattering (RLS) spectroscopy was used to investigate the complexation behavior and efficacy of the DMAPA-Glyp derivatives and plasmid DNA (pDNA). Results indicated that the DMAPA-Glyp derivatives and pDNA molecules began to aggregate and form complexes at a weight ratio of 2. Following further increases in the DMAPA-Glyp derivatives (w/w >2), pDNA was condensed to form compact complexes. RLS spectroscopy is thus a useful tool for studying the complexation behavior and efficacy of cationic polymers with nucleic acid molecules. Circular dichroism analysis further revealed that the conformation of pDNA transformed from B-form to C-form and A-form during the complexation process.