A new simple nonlinear viscoelastic constitutive model under uniaxial tension is proposed by means of experimental method and internal-variable theory．The tension tests of two viscoelastic materials

i．e．

polypropylene (PP) and high-density polyethylene (HDPE)

are carried out at different strain rate．The experimental date shows that the relation between stress and strain depends distinctly on strain rate．The result of curve fitting for experiment data indicates that the relaxation time of the materials is rate sensitive．When strain rate changes in orders of magnitude

the value of relaxation time varies in the same order．Under the condition of uniaxial stress

a new nonlinear viscoelastic constitutive relation with iteration form is studied by adopting internal-variable theory

and the convergence condition is derived．If the degree of iteration is taken to be one

then the corresponding constitutive relation reduces to Maxwell model with rate related relaxation time．Such a nonlinear viscoelastic model can be used to fit and predict the constitutive relation of PP and HDPE perfectly．