A viscoelastic theory based on a simple model is developed to describe the stress–strain relation of amorphous plastics above the glass temperature. One of the bases of the theory is that the stress is proportional to the number of molecular chains traversing a unit cross section. Experimental results for the stress–strain relation of amorphous polyethylene terephthalate in the range 80–120°C. are presented. These are shown to agree with the theory within the experimental error.