This work describes the synthesis and adsorption behavior of Fe3O4 magnetic nanoparticles (MNPs) modified with 3-glycidoxypropyltrimethoxysilane (GPTMS) and poly-lysine (P-Lys). The magnetic nanoparticles were characterized by infra-red spectra, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis and X-ray photoelectron spectroscopy. To understand the adsorption mechanism of the MNPs, we investigated anionic dye uptake capacity of Fe3O4@GPTMS@P-Lys as a function of contact time, dye concentration and pH. The MNPs could effectively remove anionic dyes including methyl blue (MB), orange I (OR-I), amaranth (AM) and acid red 18 (AR-18) from water solution. Furthermore, the improved adsorbent showed convenient separation and efficient reusability, and enhanced the adsorption capacity for anionic dyes. (C) 2014 Elsevier B.V. All rights reserved.