Constitutive equations are derived for polymers subjected to stress-induced crystallization at non-isothermal three-dimensional loading. A polymeric material is treated as an isotropic permanent network of long chains, partially transformed into crystallites while stretched. Thermodynamic potentials of a network are calculated based on the Flory concept, and stress-strain relations are developed using the laws of thermodynamics. Adjustable parameters of the model are determined for poly(ethylene terephthalate) in uniaxial extension. It is demonstrated that the constitutive relations correctly predict experimental data in uniaxial tensile tests with relatively large rates of strains.