This investigation was carried out to elucidate the influence of oxidizing atmosphere in the annealing process for making thermistor. Starting sample was prepared by mixing the nitrates of Mn, Co, Ni with molar ratio of 3.0:1.9:1.1. After fired at 1400°C, the sample was cooled to room temperature in the furnace. The oxide thus obtained was annealed in different atmospheres at 310° and 370°C. Electric conductivity change during the annealing process was followed for 15 days in the atmospheres of argon, air and oxygen. Electric conductivies changed depending on the type of atmospheres. In the oxidizing atmospheres, such as air or oxygen, electric conductivities decreased markedly with increasing annealing time at 370°C, whereas no detectable change was observed in the atmosphere of argon. No significant changes in the thermistor constants, however, were observed during the annealing process. X-ray diffraction study was made on the samples annealed in the atmospheres of argon and air. All the samples consisted of rock salt, cubic spinel and tetragonal spinel structures. A remarkable increase in axial ratio, c/a, in the tetragonal spinel structure was observed with increasing annealing time, particularly in the atmosphere of air at 370°C. The change in axial ratio, c/a, in the tetragonal spinel structure seems to be related to the electric conductivity change, because, when c/a exceeded 1.1, significant changes in the electric conductivities were observed. It can be assumed that changes of axial ratio in the tetragonal spinel structure were attributed to the Jahn-Teller distortion which was induced by the increase in Mn³⁺ concentration in the octahedral site with increasing annealing time.