Abstract: 22MnB5 ultra high strength steel was austenitizated at different heating rates using a Gleeble 3500 thermo-mechanical simulator. The effects of heating rates on austenitization temperatures of Ac1, Ac3 and on austenitization time were investigated, and the relational expression between heating rate and Ac1 was built up. The results show that both the Ac1 and Ac3 of the steel increased with the increase of heating rates. With the increase of heating rates, the temperature Ac1 increased rapidly with the heating rate below 25 ℃·s-1 and increased slowly then tended to a constant with the heating rates over 25 ℃·s-1. Meanwhile, the austenitization temperature range values (Ac3-Ac1) increased and austenitization time interval values decreased with the increase of the heating rates.