Abstract: The superplastic constitutive relation developed by the second author of the paper considers three types of microscopic deformation mechanisms and the grain growth process. However, the material parameters are difficult to identify by experiment only. By means of genetic algorithm and Levenberg-Marquardt algorithm, the material parameters included in the superplastic constitutive relation are identified. An example of Ti-6Al-4V alloy is given in this paper. The calculated curves are in agreement with those of the experiments very well. The volume fraction variations of microscopic deformation mechanisms corresponding with strain, strain rate and grain size are discussed. It is found that the contribution due to diffusive creep deformation mechanism can be neglected in most cases.