Three kinds of cobalt particles with different morphologies and crystal structures were synthesized via reducing cobaltous sulfate (CoSO4 center dot 7H(2)O) by hydrazine hydrate (N2H4. H2O) under ultrasonic wave, in which seignette salt (C(4)H(4)KNaO(6 center dot)4H(2)O) or C602H5Na3.2H20 acted as complexing agent, C16H33(CH3)3NBr acted as surfactant agent and sodium hydroxide (NaOH) acted as pH regulator. Less saturation magnetization and more coercivity were obtained compared to single hcp-cobalt and bulk cobalt, respectively. The electromagnetic properties of cobalt particles dispersed in paraffin (70 wt.%) were measured in the microwave frequency range of 1-18 GHz. The three kinds of cobalt particles all have multi-nonlinear dielectric resonances due to atomic and electronic polarization. The cobalt particles composed by nanosheets exhibit higher real part of permeability in the range of 10-18 GHz compared to spherical cobalt particles. The reflection loss values of cobalt particles were calculated according to the transmission-line theory, and the minimal reflection loss of 19.06 dB at 17.42 GHz was observed corresponding to a thickness of 5 mm. (C) 2014 Elsevier B.V. All rights reserved.