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Superplasticity of TA15 alloy was investigated based on the variable strain rate sensitivity exponent m value method. The superplastic tensile tests were conducted at 1053~1223 K. Results show that the elongation of TA15 alloy is 580%~1922%. The microstructure analysis shows that the grains are coarsened with increasing the temperatures but remain the equiaxed structure during the deformation. In addition, the phase transformation α→β occurs at 1223 K, resulting in the serious reduction in superplastic property. Compared with that treated by the constant strain rate method, the superplasticity of TA15 alloy treated by the variable m value method is greatly enhanced. During the superplastic deformation, the mechanical properties and microstructure evolution are in good agreement with the Ashby-Verrall model. Therefore, it can be inferred that the dominant superplastic mechanism for TA15 alloy based on variable m value method is the grain boundary glide accommodated by diffusion creep.