Different-sized Fe3O4 nanospheres (from similar to 30 nm to similar to 100 nm) were prepared via a solvent-thermal route by controlling the content of FeCl3 center dot 6H(2)O in the synthesis. The crystalline structure, surface morphology and magnetic properties of obtained Fe3O4 nanospheres were systematically characterized. Additionally, the complex permittivity and permeability were measured to simulate the electromagnetic absorption of different-sized Fe3O4 nanospheres from 0.5 to 15 GHz. The results indicated that the Fe3O4 nanospheres with an average size of similar to 65 nm exhibited the strongest magnetic properties (saturation magnetization of 89.24 emu/g) and electromagnetic absorption (reflection loss of -30.37 dB at 9.68 GHz). (C) 2014 Elsevier B.V. All rights reserved.