The study on novel physical properties of structural ceramics or ceramic composites could make them more conducive to be function- and structure-integrated materials. Herein, titanium nitride-alumina (TiN-Al2O3) duplex ceramics were prepared and the dielectric spectra of the ceramics were studied from 10 MHz to 1 GHz. Negative permittivity appeared when TiN content exceeded 40 wt% due to the induced plasmonic state of massive delocalized electrons in connected TiN grain networks. Meanwhile, alternating current conduction behaviors of the duplex ceramics were discussed with percolation theory. Furthermore, the analysis of reactance by equivalent circuit models indicated that negative permittivity ceramics exhibited inductive character. This work realized negative dielectric behaviors in TiN-Al2O3 duplex ceramics and would promote the study of electromagnetic functionalization in wave shielding or attenuation for structural ceramics.