The effect of deformation temperature on the mechanical properties of 1-GPa-grade TRIP steels with different retained austenite (γ_R) morphologies was studied. The temperature dependence on the deformation-induced martensitic transformation behavior was also investigated. The uniform elongation below room temperature was relatively large in the needle-like γ_R steel, whereas the tensile strength at each temperature was almost the same as it was independent of the γ_R morphology. The better tensile strength–uniform elongation balance was obtained at 373 K for the needle-like γ_R steel and at 473 K for blocky γ_R one. The mechanical stability of γ_R was higher in the needle-like γ_R steel, according to the x-ray diffraction experiments. γ_R was mechanically stable with increasing temperature, but its volume fraction decreased at temperatures above 473 K because of the deformation-induced bainitic transformation. In this paper, the quantitative conditions of deformation-induced transformation to obtain better uniform elongation in the 1-GPa-grade TRIP steels are summarized from the viewpoints of the volume fraction of deformation-induced martensite and the transformation rate.