Abstract: Coming out from the mutual polarization between the lanthanides metal ions and the water polarized molecules,and considering the relation among polarization and the effective nuclear charge number Z^* of the lanthanides metal ions and the number m of the 4f electrons, this paper has given a half-empirical formula for calculating the hydrate energies as follows: L=a(Z²_*/r)+6m+c According to this formula the hydrate energies of the lanthanides metal ions are calculated with the results being satisfactory.