Abstract: The microstructural evolution of undercooled austenite in C-Mn-Si TRIP steel and C-Mn-Al-Si TRIP steel during hot deformation was investigated by hot uniaxial compression testing on a Gleeble-1500 hot simulator, and the influences of alloying elements and processing parameters on the dynamic transformation of undereooled austenite were analyzed. It is found that the dynamic transformation kinetics of undercooled austenite in C-Mn-Si TRIP steel and C-Mn-Al-Si TRIP steel is faster obviously than the isothermal transformation kinetics of undercooled austenite. In comparison with C-Mn-Si TRIP steel, the A₃ temperature of C-Mn-AI-Si TRIP steel is elevated markedly by the substitution of 1% Al for Si, and the dynamic transformation of undercooled austenite in C-Mn-AI-Si TRIP steel is faster at the same processing conditions. But the averave size of ferrite grains fomred by the dynamic transformation in C-Mn-Si TRIP steel is finer. The refinement of austenite grains before the dynamic transformation is of benefit to the development of the dynamic transformation of undercooled austenite. Increasing the deformation temerature or the strain rate of undercooled austenite during hot deformation has a retarding effect on the dynamic transformation of undercooled austenite, but this effect is not marked.