Annealing enlarges the grain size and releases the large microstrain in the Ni/TiO₂ nanocomposite prepared by mechanochemical synthesis. This greatly enhances the dielectric resonance and a clear Cole–Cole semicircle is observed for the annealed sample, for which a maximum of reflection loss (RL) as large as –45.2 dB at 12.6 GHz with RL values exceeding −10 dB in the whole Ku-band is obtained for a layer of 2.3 mm thickness. In particular, a 20 dB bandwidth over the frequency range from 16.4 to 18 GHz with a RL value of 41 dB is found at 17.1 GHz for 2.1 mm thickness. The great improvement of microwave-absorption properties after annealing can be ascribed to an enhanced dielectric resonance and an excellent match of magnetic and dielectric loss.