According to the phenomenon theory of micellar solution, the Gibb's free energy of salt added C8-lecithin micellar solutions is deduced using Debye-Huckel's and Kirkwood's theories of electrolytes. The mechanism of ion-amphiphilic molecule interaction, and the variation of the critical temperature of phase separation in the solutions vs ionic strength are also described with the Gibb's free energy model. The model consists of a linear term of ionic strength, contributed from ion-dipole interaction, and a square root term of ionic strength, contributed from ion-ion interaction. A formula which describes the critical temperature of phase separation is also deduced from the Gibb's free energy model. The formula appears as, T’c=Tc- A1I1/2+A2I ,and agrees well with the experimental data of both monovalent and divalent salt added micellar solutions.