Structure and electrical properties of NiMg x Mn 2−x O 4 (where x=0, 0.1, 0.2, 0.3 and 0.4) ceramics prepared by using sol–gel derived powders have been studied. A careful control of the synthesis process enables producing NiMn 2 O 4 phase with spinel structure at a relatively low calcining temperature of 600°C. All compositions showed a linear increase in lnρ vs. 1/T over the measured temperature range, indicative of negative temperature coefficient (NTC) characteristics. As the concentration of Mg increased from 0.0 to 0.4, the ρ 25 , B 25/85 stability constant and activation energy of NiMg x Mn 2−x O 4 thermistors increased from 4026 to 11,943Ωcm, 3825–4144K and 0.329–0.357eV, respectively. In addition, the ΔR/R value first reduced fleetly from 0.78% to 0.32% and then decreased slowly to 0.12%.