Abstract: The influence of 0.21% Al (mass fraction) on the kinetics of austenite decomposition in microalloyed steel containing 0.05%C-1.7%Mn-0.4%Si-0.05%Nb-0.03%Ti was studied. The results show that the hardenability of the experimental steel increases apparently by adding 0.21% Al when the cooling rate is in the range of 1~30 ℃/s. For isothermal treated plates at 600 ℃ for 20 min after rolling process, the banding microstructure of the plates containing 0.21% Al is abated or avoided and the overall mechanical properties of the plates is promoted greatly.