Due to the strain-induced martensitic transformation during plastic deformation, a transformation-induced plasticity (TRIP) steel possesses such favorable mechanical properties as high strength, ductility and toughness. Since its favorable mechanical properties are realized under quite restricted circumstances, the prediction and control of the deformation processes is indispensable to determine the expected mechanical properties of TRIP steel. However, it is very difficult to obtain the TRIP steel which only have the martensitic structure before deformation and the stress-strain relationship of each phases under various temperatures cannot be measured by the experiment. So the identification of the parameters in the constitutive equations of TRIP steel is necessary. Here, a method employing a finite-element simulation and the non-linear least square method with constraint conditions was proposed to identify the constitutive parameters of TRIP steel from the true stress-plastic strain and the volume fraction of martensite-plastic strain relations obtained by the uniaxial tensile test and computational simulations. Then, the application of this procedure employing virtual data of an ideal TRIP steel which possesses more favorable mechanical properties is discussed for investigating improvement of the mechanical properties of TRIP steel.