Magnetic fluids based on superparamagnetic, well-crystalline and mono-dispersed Fe3O4 nanoparticles are synthesized by a facile thermal decomposition method. Low-cost Fe(oleate)(3) is used as ferrous resource, and paraffin oil is introduced as solvent and carrier simultaneously. Fe3O4 nanoparticles with three kinds of different size (12, 16 and 20 nm), exhibiting different saturation magnetization of 65.51, 68.03 and 74.48 emu/g, respectively, are prepared easily by controlling the thermal decomposition time. Magnetic performance and viscosity-temperature characteristics of the magnetic fluid are also studied. It is found that the saturation magnetization of the magnetic fluid with 5% Fe3O4 mass percentage is 2.94 emu/g, which exhibits a good viscosity temperature characteristics compared with paraffin carrier. (C) 2014 Elsevier Ltd. All rights reserved.