Colloidal particles with magnetic properties have become incresingly important both technologically and for fundamental studies. Here, chemical initiator-free miniemulsion polymerization of styrene and butyl acrylate has been performed for preparation of magnetic nanocomposite particles with the diameter of 81-150 nm in the presence of sodium dodecyl sulfate as surfactant, span 80 as stabilizers, and hexadecane as hydrophobe. The polymerization reaction was initiated and progressed under ultrasonic irradiation, generated by immersed probe into the latex. The key point in achievement of encapsulation of modified Fe3O4 nanoparticles of a stable colloidal dispersion at the end of the reaciton. The obtained produces in each step were characterized by FTIR spectroscopy. Dynamic light scattering analysis was used to follow particles and formation of core-shell structure were analyzed by SEM and TEM micrographs, respectively. TGA and magnetometry of the polymeric films confirmed the extent of insertion of used magnetite and their corresponding behaviour. (c) Wiley Periodicals, Inc.