A nonlinear viscoelastic constitutive equation of glassy polymers has been developed on the basis of the internal time and irreversible thermodynamics of internal variables. The constitutive equation is a generalization of spring-dashpot models and describes the yield behavior of glassy polymers. With this constitutive equation, the rate of entropy production has been determined to show a peak near the yield. We propose a new yield criterion, that yield occurs at the maximum rate of entropy production. The yield criterion is almost equivalent to that of peak stress and a thermodynamic interpretation of yield. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2400-2411, 2003.