For the design of structural parts made from plastics, the long term mechanical behaviour is of major influence. A phenomenological model will be introduced that simplifies the calculation of the three-dimensional creep deformation of structural components under temperature influence. The existence of a creep potential is assumed that takes into account the compressibility and the differing behaviour of the material under tensile and compressive stresses. for the equivalence of the multi-axial stress state and the one dimensional stress state one postulates a potential as a function of the equivalent stress. The free parameter of the potential is expresses by the temperature dependent Poisson's ratio.