Abstract: The general form of the free energy and its criterion is educed from the energy postulate, and the essential connotation of the free energy and its criterion is discussed. It is considered that the general form of the free energy is summation of all energy forms correlating with N kinds of work in a thermodynamic system and that the inreversible processes of an isolated system for ever results in the reduction of total free energy under the condition of the fixed temperature. It is showed by an example how the general form of the free energy criterion is applied, which embodies the philosophy thought that all kinds of movement may interact.

Key words: energy postulate, free energy criterion, water drop increase