Abstract: An alloy phase stability index diagram comprising a phase distribution range specified therein by calculating average values Md and Bo of an alloy according to the following formulae with respect to an energy level of "d" orbitals of an alloying element and a bond order between a mother metal and an alloying element:Md=.SIGMA.Xi(Md)i (1)Bo=.SIGMA.Xi(Bo)i (2)where Xi is an atomic fraction of the alloying element (i) and (Md)i and (Bo)i are the Md value and the Bo value, respectively, and by locating alloys with known compositions in the index diagram in which Bo or Md thus defined is taken in an ordinate or an abscissa or Bo and Md are taken in both the coordinates.