A sonochemical technique is used for in situ coating of iron oxide (Fe3O4) nanoparticles on outer surface of MWCNTs. These Fe3O4/MWCNTs were characterized using a high-resolution transmission electron microscope (HRTEM), X-ray diffraction, and thermogravimetric analysis. The as-prepared Fe3O4/MWCNTs composite nanoparticles were further used as reinforcing fillers in epoxy-based resin (Epon-828). The nanocomposites of epoxy were prepared by infusion of (0.5 and 1.0 wt %) pristine MWCNTs and Fe3O4/MWCNTs composite nanoparticles. For comparison purposes, the neat epoxy resin was also prepared in the same procedure as the nanocomposites, only without nanoparticles. The thermal, mechanical, and morphological tests were carried out for neat and nanocomposites. The compression test results show that the highest improvements in compressive modulus (38%) and strength (8%) were observed for 0.5 wt % loading of Fe3O4/MWCNTs. HRTEM results show the uniform dispersion of Fe3O4/MWCNTs nanoparticles in epoxy when compared with the dispersion of MWCNTs. These Fe3O4/MWGNTs nanoparticles-infused epoxy nanocomposite shows an increase in glass transition (T-g) temperature. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2783-2792, 2010