To develop super-high strength TRIP steel for automobile application with good combination of strength and plasticity,an ultra-high-strength TRIP steel 0.2%C-2.0%Si-1.8%Mn was investigated at different intercritical annealing temperatures and bainitic isothermal temperatures.Microstructures were observed by scanning electron microscopy,transmission electron microscopy,electron backscatter diffraction,and X-ray diffraction,and mechanical properties were tested under a uniaxial tensile testing machine.It shows that the best comprehensive mechanical properties were obtained for the experimental steel particularly annealed at 820 ℃ and austempered at 410 ℃.Yield strength of the tested steel mainly depends on ferrite,while the high strength of the tested steel is related to martensite which forms through blocky austenite transformation during cooling and subsequent tensile test attributed to the TRIP effect.Hence,it could be concluded that the grain size of the retained austenite and carbon content in retained austenite are the main reasons for high elongation of the tested steel. To develop super-high strength TRIP steel for automobile application with good combination of strength and plasticity, an ultra-high-strength TRIP steel 0.2% C-2.0% Si-1.8% Mn was investigated at different intercritical annealing temperatures and bainitic isothermal temperatures. Microstructures were observed by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and X-ray diffraction, and mechanical properties were tested under a uniaxial tensile testing machine. It shows that the best comprehensive mechanical properties were obtained for the experimental steel particularly annealed at 820 ℃ and austempered at 410 ℃ .Yield strength of the tested steel mainly depends on ferrite, while the high strength of the tested steel is related to martensite which forms through blocky austenite transformation during cooling and subsequent tensile test attributed to the TRIP effect. Hence, it could be concluded that the grain size of the retained austenite and ca rbon content in retained austenite are the main reasons for high elongation of the tested steel.