The negative temperature coefficient Mn2.1Ni0.9O4 ceramics were prepared by conventional solid-state reaction method. The structure and electrical properties of the prepared ceramics were characterized by X-ray diffraction, direct current (DC) resistance temperature measurement and alternative current (AC) impedance analysis. Furthermore, the conductive mechanism of Mn2.1Ni0.9O4 ceramics were expatiated in the microscopic respect. The results show that as-prepared Mn2.1Ni0.9O4 materials are composed of phase spine and rock salt phase (NiO). The contribution to the conductivity of Mn2.1Ni0.9O4 ceramic is mainly due to the grain boundary and grain. Both grain and grain boundary components possess NTC characteristic in the measured temperature range of 293–363 K and their conductivity exhibit a good agreement with Nernst-Einstein equation, which indicate that both conductive mechanism of grain and grain boundary components could be explained by small polaron hopping. However, the NiO phase has a greater influence on the grain boundary conductivity than the grain.