Co coated Fe3O4 core/shell-type nanoparticles were fabricated by hydrothermal technique and electroless plating process. X-ray powder diffraction (XRD), X-ray fluorescence spectrometer (XRF) and transmission electron microscope (TEM) were employed to investigate the crystal structure, element composition and morphology of the prepared nanoparticles. Vibrating sample magnetometer (VSM) and vector network analyzer were used to measure the magnetic properties and electromagnetic parameters of pure Fe3O4 and Fe3O4/Co core/shell-type nanoparticles, then reflection losses (RL(dB)) were calculated in the frequency range of 2-18 GHz. Magnetic studies revealed typical ferromagnetic behavior for the pure Fe3O4 and Fe3O4/Co core/shell-type nanoparticles with their saturation magnetization (M-s = 63.1 and 72.4 emu/g) and coercivity (H-c = 99.5, and 165.4 Oe), respectively. Due to the existence of the core/shell structure, the electromagnetic characteristic of the Fe3O4/Co nanoparticles exhibit better microwave absorption performance than the pure Fe3O4 in the range of 2-18 GHz, such as more powerful absorbing property and wider frequency band of microwave absorption. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.