Negative temperature coefficient (NTC) thermistors are ceramic semiconductors derived from metal oxides such as Mn, Ni, Co, Cu, Cr, Fe etc. The electrical resistance of NTC thermistors decrease with increasing temperature. In industrial applications, the desired material constant (B) of NTC thermistors is in the range of 2000 to 5000 K and the sensitivity coefficient (α) is in the range of (-2.2) to (-5.5) %/K. The electrical properties of NTC thermistors can be changed significantly by the type and molar ratio of metal oxide additions as well as the selected process parameters during manufacturing of samples. In this study, the effect of Cr2O3 addition on the electrical properties of Mn-Ni-Co-O NTC thermistors sintered at 1100 oC for 5 hours was investigated. The electrical resistance was measured in a temperature programmable furnace between 25 and 85oC in steps of 0.1oC. The material constant “B”, the activation energy “Ea” and the sensitivity coefficient “α” values were calculated. The aging behaviour of Mn–Ni–Co–Cr-O NTC ceramics was also investigated by electrical resistivity measurement after accelerated aging.