Abstract: The austenite grain sizes of low carbon bainite steel with high Nb content after solid solution at different temperatures from 950 ℃ to 1 270 ℃ for different time from 30 min to 150 min were determined by metallographic method and the growth behavior of the austenite grains was studied. The solid solution Nb content in the tested steel was calculated, and then the mathematical model for the austenite grain growth was established and verified by the experiments. The results show that the austenite size increase value in the temperature range 1 220-1 270 ℃ was maximum and followed by that in the range 1 100-1 150 ℃; The grain size increase value in the temperature range 950-1 000 ℃ was larger than that in the range 1 000-1 050 ℃ due to the rapid dissolution of the alloying cementite in the experimental steel at the temperature of 1 000 ℃.The solid solution Nb content calculated at 1 150 ℃ reached 0.088wt%, and by this time the most NbC particles were dissolved, resulting in the rapid growth of the austenite grains in the temperature range of 1 100-1 150 ℃. In the range 950-1 270 ℃, the activation energy for austenite grain growth was 154.4 kJ·mol-1. The austenite grain sizes calculated by the as-established grain growth model of the tested steels were in accordance with the measured values.