We explored the removal of cationic dyes - crystal violet, methylene blue and alkali blue 6B - from water solution by adsorption with magnetic nanoparticles (MNPs) modified with 3-aminopropyltriethoxysilane and copolymers of acrylic acid and crotonic acid. The noval magnetic nanoparticles was characterized by transmission electron microscopy, X-ray diffraction, infra-red spectra and thermogravimetric analysis. The cationic dyes could be quickly removed from water solution with high efficiency at pH 5-12. More significantly, the MNP showed high efficiency as a reusable adsorbent for fast and convenient removal of cationic dyes from water solution. (C) 2012 Elsevier B.V. All rights reserved.