Abstract: The concept of "the third metastable zone" of crystallizaton from aqueous solution is proposed in this paper. The width of the metastable zone is found much wider than the first one and the second one by measuring a few kinds of aqueous solution of salts. In addition, the solid -liquid interfacial tension σ, the radius of critical nucleus r* and the number of molecules in a critical nucleus i* are obtained by processing the experiment data with classical homogeneous nucleation theory.