The microstructure and mechanical properties of hot rolled C-Mn-Si and C-Mn-Al-Si TRIP (transformation induced plasticity) steels based on dynamic transformation of undercooled austenite (DTUA) were investigated by uniaxial hot compression tests in a Gleeble-1500 simulation test machine. The results show that in the multiphase microstructure of the investigated steels, the grain size of ferrite is fine, the bainite packets are small and have chaotic orientations, the retained austenite distributes uniformly and has high volume fraction due to the DTUA. Moreover, due to lots of grain boundaries of ferrite and bainitic ferrite, high density of dislocation in bainitic ferrite and a large amount of granular retained austenite located among/between ferrites, the steels have excellent mechanical properties, which is characterized by a combination of continuous yielding, high strength (about 890 MPa for C-Mn-Si steel and 760 MPa for C-Mn-Al-Si steel) and high elongation (26% and 32% respectively) as well as low yield/strength ratio.