Cellulose nanocomposites containing high contents of Fe3O4 nanoparticles. were Successfully prepared with regenerated cellulose films as a matrix and mixture solutions of Fe2+/Fe3+ as precursors. The structure and properties of the magnetic nanocomposite films were investigated with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravmetric analysis, and vibrating sample magnetometry. Fe3O4 nanoparticles as prepared were irregular spheres and were homogeneously dispersed in the cellulose matrix. With an increase in the concentration of precursors from 0.2 to 1.0 mol/L, the content of Fe3O4 nanoparticles in the dried nanocomposites increased from 12 to 39 wt %, and the particle diameter increased from 32 to 64 nm. The Cellulose nanocomposite films demonstrated superparamagnetic behavior, and their saturation magnetizations were in the range 4.2-21.2 emu/g, which were related to the increase in Fe3O4 nanoparticle content. With increasing nanophase content, the nanocomposite films displayed significantly anisotropic magnetic properties in the parallel and perpendicular directions. This study provided a green and facile method for the preparation of biobased nanocomposite films with high nanophase content and excellent magnetic properties. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2477-2484, 2009