Experimental data are presented on thermal field-flow fractionation (TFFF) of anionic polystyrene (PS) samples in the rangeM= (4–12)·106. They show extensive degradation of macromolecular chains at relatively low rate gradients (G< 30 s−1). The possibility of the influence of relaxation effects on the shape of fractograms and the elution volumes of the samples was taken into account. Mean concentrations in accumulative zones were evaluated. It was shown that, in all cases when degradation was observed, the accumulative zones are the layers of entangled macromolecules. The use of the scaling approach made it possible to simulate layer extension toward the channel center under the influence of the rate gradient. It was shown that, during stretching, the layer is destroyed into blobs, the size of which is determined by experimental conditions. An expression for the critical gradient leading to layer degradation was derived. Quantitative evaluations of fragment sizes and critical gradients obtained from the model are in good agreement with experimental data. The model developed for specific experimental conditions confirms the proposed general mechanism of the so-called shear degradation of macromolecules. The physical picture of degradation in the TFFF channel was considered.