A one-step method of preparing Fe3O4 magnetic nanoparticles coated with carboxymethyl chitosan was evaluated. The coated magnetic nanoparticles showed high saturate magnetism strength (ós = 37.5 emì/g). The diameter of modified nanoparticles was approximately 14.5 nm. Furthermore, the binding between modified magnetic nanoparticles and DNA was examined by agarose gel electrophoresis. The preliminary results demonstrated that carboxymethyl-chitosan-modified Fe3O4 magnetic nanoparticles might be a potential DNA carrier.