Abstract: To characterize the behavior of polymers and the mutual influences among moisture, temperature, and stress in polymers, a constitutive equation for isotropic viscoelastic materials, with a coupling among behaviors of moisture diffusion, heat conduction and mechanical deformation, is developed using the Helmholtz free energy and irreversible thermodynamics. In the equation, internal variables are introduced to describe the viscosity of the materials. It is shown that through simplification some currently used computational models can be generated from the equation proposed in this paper.