Synthesis of Fe3O4/polyaniline (PANI) core/shell hybrid microspheres through an in situ polymerization route with the 300 nm Fe3O4 microspheres as the cores and nucleation sites for PANI is reported. PANI shell thickness, from 30 nm to 120 nm, can be controlled by modulating the weight ratio of aniline monomer and Fe3O4 microspheres. Fe3O4 microspheres that are actually comprised of many small nanoparticles display super paramagnetic feature, which has been maintained in Fe3O4/PANI core/shell hybrid materials. Fe3O4/PANI core/shell hybrid materials have enhanced microwave absorption than both the Fe3O4 microspheres and PANI, where a maximum reflection loss of −37.4 dB at 15.4 GHz has been reached from the sample with a PANI shell thickness of 100 nm. Introduction of dielectric loss, interfacial loss, and improved impedance from coating the Fe3O4 microspheres with PANI should account for the improved microwave absorption properties of the prepared Fe3O4/PANI core/shell hybrid materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1909–1916, 2013