Stability against chemical reduction is one of most important material property for high temperature thermistor materials. In this paper, material design of spinel type MgCr₂O₄ by first principles calculation in collaboration with experiments was discussed. MgCr₂O₄ is known to show Cr-deficit p-type semi-conduction in the elevated temperature region. The theoretical calculations of Cr-vacancy formation energy showed preferred formation of Cr-vacancies and their stability in oxidizing atmosphere, while their instability in reduction atmosphere. The stable semi-conducting Po₂ region (i.e. 1 to 10^-10 atm at 1000°C) of MgCr₂O₄ was determined by precise experiments. First principles calculation can be a powerful tool for the ceramic material design, when they are used in collaboration with precise experiments.