Fe3O4 nanoparticles were synthesized by hydrothermal method with the synergy reduction of ethylene glycol (MEG) and diethylene glycol (DEG). The purity, grain size, magnetism and the microwave absorption performances of the samples can be controlled by the concentration of DEG in the precursor solution. Under the optimized condition of synthesis, the product is of highly crystallized cubic Fe3O4 and the crystallite size of Fe3O4 is about 44-60 nm with the saturation magnetization about 96 emu/g. The Fe3O4 paraffin composites exhibit excellent microwave absorption properties at the frequency range of 1-18 GHz, which are attributed to the electron transition resonance, natural resonance and polarization of Fe3O4. The minimum reflection loss of Fe3O4 synthesized under the MEG and DEG content of 3.58 mol/l and 2.10 mol/l, respectively, can reach -42 dB at a thickness of 2.1 mm. And the effective absorption bandwidth of the samples can reach 3.9 GHz (7.3~11.2 GHz) at the thickness of 2.5 mm. The result demonst...