Ni nanocapsules with a core of Ni nanoparticle and a shell of amorphous Al/AlOx have been prepared by the arc discharge method. The natural resonance in the Al/AlOx-coated Ni nanocapsules shifts to 6.4 GHz, due to the surface anisotropy of Ni nanoparticles. An optimal RL value of -40.96 dB was observed at 9.2 GHz on a specimen with a matching thickness of 2.40 mm. Theoretical simulation for the microwave absorption using the transmission line theory agrees reasonably well with the experimental results. The frequency of microwave absorption complies with the quarter-wavelength matching model. (c) 2013 Elsevier Ltd. All rights reserved.