Failure during superplastic deformation (SPD) may result from geometrical macroscopic instabilities and/or microstructural aspects. However, the available failure criteria are based either on geometrical instabilities or microstructural features and do not account for both failure modes. In this work, a new multiscale stability criterion is developed by combining a modified microstructure-based constitutive equation with grain and cavitation evolution equations. The new criterion is used to design optimum variable strain rate forming paths for Ti-6Al-4V SP alloy. The effects of void fraction, grain size and strain rate sensitivity on the optimum forming paths are also examined. In addition, FE analysis of superplastic blow forming of a hemisphere is carried out using the optimum forming path and the results are compared with those obtained using constant strain rate forming paths. It is shown that the proposed multi-scale criterion can reduce the forming time without causing localized deformation and thinning.