Strain rate sensitivity index m is one of the vital mechanical parameters for deter- mining material superplasticity. In this paper, the existing formulae for measuring m value are reviewed, and it is found that the m values can be classified into three classes: mi under constant length, mv under constant velocity, and mP under con- stant load. The constraint equation of the generalized m value is established ac- cording to the tensile constitutive equation and the basis theory for plastic me- chanics. Based on three typical deformation paths, the m value is redefined. Fur- thermore, from the formula of generalized m value, the formulae for measuring mi, mv and mP are theoretically deduced. The precise methods with numerical simula- tion are presented. The results prove that the m value is a non-constant and its dependence on ε changes with the deformation path. Under different deformation paths, the m values calculated from the same formula are different. Using different formulae, the m values under the same deformation path are also different. There- fore, deformation path and corresponding formula should be given during the measurement of the m value. Moreover, it is explained theoretically and experi- mentally that why the mv value under constant velocity is sometimes negative but the mP value under constant load is sometimes lager than 1. The aim of the analysis and measurement of the m value is to facilitate the study on the relationship be- tween macroscopical mechanical laws and microscopic physical mechanisms during superplastic deformation. Strain rate sensitivity index m is one of the vital mechanical parameters for deter- mining material superplasticity. In this paper, the existing formulae for measuring m value are reviewed, and it is found that the m values ​​can be classified into three classes: mi under constant length, mv under constant velocity, and mP under con- stant load. The constraint equation of the generalized m value is established ac- cording to the tensile constitutive equation and the basis theory for plastic me- chanics. Based on three typical deformation paths , the m value is redefined. Fur thermore, from the formula of generalized m value, the formulae for measuring mi, mv and mP are are theoretically deduced. The precise methods with numerical simula tion are presented. The results prove that the m value is a non-constant and its dependence on ε changes with the deformation path. Under different deformation paths, the m values ​​calculated from the same formula are different. Using different formulae, th there are fore-deformation path and corresponding formulas should be given during the measurement of the m value. Moreover, it is explained theoretically and experi- mentally that why the mv value under constant velocity is sometimes negative but the mP value under constant load is at lager than 1. The aim of the analysis and measurement of the m value is to facilitate the study on the relationship be- tween macroscopical mechanical laws and microscopic physical mechanisms during superplastic deformation.