Mechanical milling (MM) is a non-equilibrium processing technique, which can provide new materials with metastable structures. However, plausible explanation and prediction of the phase formation during MM is still difficult due to its dynamic nature. In this study, to develop the criteria of the phase stability during MM, two indicators, the molar atomic volume (Vatom) and the number of atoms in the reduced unit cell (Z atom), were proposed, and the validity of them was investigated by plotting milling-induced polymorphic transformations reported in literatures for the variation of V atom and Zatom. As a result, phases with the smaller Vatom or Zatom values were found preferable during MM. © Springer 2010.