To interpretation the overall stress relaxation behaviors of drawn polyester film in solvents(NaOH solution (3$0^{circ}C$ and 8$0^{circ}C$) and ionic solutions (KCl and CaCl2)), 5-element mechanical model and its constitutive equations were proposed. And polynomical curve fitting method was chosen as an optimum analytical method and used for the examination of correspondence those equations for overall relaxation process to the experimental data. The polynomial curve fitting program was applied to the interpretation of these relaxation behaviors. As a result, it was shown that the application of the constitutive equations for 5-element mechanical model to the experimental data was reasonable.