Cationic magnetic nanoparticles are prepared by covalently binding spermidine and polyethylenimine onto the surface of nanoparticles via a glutaraldehyde coupling method. Nanoparticles modified by spermidine or polyethylenimine were characterized using Fourier-transformed infrared spectra, transmission electron microscopy and scanning electron microscopy. In this study, cationic magnetic nanoparticles were prepared by covalently conjugating cationic ligands onto the surface of nanoparticles. The plasmid DNA adsorption by the surface-modified magnetic nanoparticles was analyzed by Freundlich, Temkin, and Langmuir models. The maximal adsorption capacities in Langmuir model for polyethylenimine- and spermidine- modified nanoparticles are 341 and 116 μg/mg, respectively. Overall, the results demonstrated that the polyethylenimine-modified magnetic nanoparticles has the potential for purification of plasmid.