The isothermal grain growth behavior for a new ultrahigh-strength stainless steel (UHSSS) is investigated in temperature range from 900 to 1150 ℃ and holding time range from 0 to 20 min. In the temperature range from 1000 to 1050 ℃, a bimodal grain size distribution was induced by different austenite grain growth rates which resulted from the weakened pin-ning effect by the partial dissolution of M6C particles along austenite grain boundaries. Further raising heating temperatures, M6C particles almost dissolved and the bimodal grain size distribution phenomenon became weakened, indicating that the austenite grain coarsening temperature of the new UHSSS was close to 1050 ℃. According to the present experimental results, a pragmatic mathematical model based on the Arrhenius equations was developed to predict the austenite grain growth process, which elaborated the influence of heating temperature, holding time and initial grain size on the austenite grain growth. Predictions for the new UHSSS presented a good agreement with experimental results.