Abstract: By using the tensor analysis, the paperproves that the three theories for describing the nonlinear constitutiverelation of incompressible viscoelastical materials,including BKZ theory, Lianis theory andChirstensen theory, have a samerepresentation when they are used for the uniaxial tension of isotropicmaterials. And we also show that the three theories amount the samein explaining the nonlinear constitutive relation.