Novel hyperbranched aromatic polyamide coated Fe3O4 magnetic nanoparticles were prepared by mixing hyperbranched aromatic polyamide/N,N-dimethylacetamide (DMAC) solution with Fe3O4 nanoparticle/DMAC suspension. The resultant materials were characterized by transmission electron microscope (TEM), Fourier transform infrared (FTIR) spectrometry, UV-Vis transmission spectrometry, superconducting quantum interference device (SQUID) magnetometer, wide angle X-ray diffraction (WAXD) and thermogravimetric analysis (TGA). The results showed that the Fe3O4 nanoparticles were well separated by the hyperbranched aromatic polyamide and could be suspended stably in DMAC over one month, whereupon the suspension showed rapid response to an external magnetic field. The diameter of the magnetic particles was around 5-14 nm, and the material exhibited superparamagnetic properties above the blocking temperature, which indicated that the magnetic hyperbranched aromatic polyamides nanoparticles obeyed a single-domain theory. (c) 2006 Elsevier B.V. All rights reserved.