As discussed in previous chapters, the extremum of state function is the necessary and sufficient condition for equilibrium. However, the equality of the second law had been practically used as the criterion of equilibrium for about 150 years since the establishment of thermodynamics. That is, only simple systems were under consideration in classical thermodynamics. Now it is known that [di S 1<0,di S 2 >0&di S=0] belongs to the equality part of the second law, but it is a new field of nonequilibrium nondissipative thermodynamics which provides a theoretical base for nonequilibrium phase diagrams. There is at least one more dimension of nonequilibrium conditions for nonequilibrium phase diagrams in comparison with the classical or traditional equilibrium phase diagrams. Theoretically calculated nonequilibrium phase diagrams agree with experimental data of activated low-pressure diamond synthesis. The calculation principle and method of nonequilibrium phase diagrams are discussed in detail in this Chapter.