Changes in crystal structure and thermistor constant in Mn-Co-Ni oxide, a thermistor material, in cooling process below 1000°C after firing at 1400°C have been studied. The starting oxide was made of nitrates of Mn, Co and Ni with molar ratio of 3.0:1.9:1.1 by firing at 1400°C and by cooling to room temperature in the furnace. The samples used in this study were prepared by annealing the above oxide at various temperatures between 250° and 1000°C for 24 hours. Each sample was quenched rapidly in the water. X-ray diffraction study of these samples heat-treated below 900°C identified three kinds of crystal structures, such as tetragonal spinel, cubic spinel and rock salt. Among these crystal structures, no change in diffraction patterns was observed with both cubic spinel and rock salt structures. However, the axial ratio, c/a, of the tetragonal spinel, varied with increasing heat-treatment temperature, and a sharp decrease in c/a values was observed at around 600°C as shown in Fig. 4. On the other hand, the thermistor constant B values changed with increasing heat-treatment temperatures, and a remarkable depression of B values was observed between 550° and 650°C as shown in Fig. 6. This result is closely associated with the change in c/a values in the tetragonal spinal structure observed in the same temperature range.