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Taking 8Cr4Mo4Ni4V bearing steel as the experimental material, the effect of heating temperature and holding time on austenite grain size was studied by austenite isothermal experiment. The grain size under different heat treatment conditions was measured by using linear intercept method. The results show that with the increase of heating temperature and holding time, the austenite grain size increases gradually. When the heating temperature exceeds 1100℃ or the holding time is over 120min, grain coarsening begins to occur. When the heating temperature exceeds 1150℃, grains become coarsened completely. The mathematical model of the austenite grain growth of 8Cr4Mo4Ni4V bearing steel with the heating temperature and holding time is obtained by linear regression, which can provide a theoretical basis of forging and heat treatment process for the 8Cr4Mo4Ni4V aviation bearing steels.