By using the non-Newtonian, four elements and elastic-visco-plastic model, equations of the stress-strain curves of PET filaments at different strain rates were derived and discussed with experimental results. Strain rates range from 10$^$-4/ to 10$^$-1/ sec$^$-1/ and experimental results above the yield point deviate from theoretical values about 3∼15％ according to strain rates. The non-Newtonian, four elements and elastic-viscoplastic model is suitable to describe the stress-strain behaviors above the yield point of PET filaments manufactured at various conditions, and initial slope of the stress-strain curves of PET filaments is independent of strain rate.