As one of important functional materials, magnetic nanoparticles have aroused great interest of textile researchers in recent years because of their unique properties such as low toxicity, good stability, and strong magnetic response, which can endow the natural fiber with excellent separability and recyclability. [1] Herein, a facile approach has been developed for preparing Fe3O4 nanoparticle (Fe3O4NP)-coated cotton fibers by hyperbranched poly(amidoamine) (HBPAA)-induced assembly of Fe3O4NPs onto the surface of cotton fibers in aqueous solution. Fe3O4NPs were synthesized in aqueous media by using citric acid as a stabilizer. Cotton fibers were firstly treated with HBPAA and then immersing in Fe3O4 aqueous solution.