The temperature, the stem length and the strain-rate dependence of the yield stress of polyethylene (PE) is investigated via a modified crystal plasticity approach. Yielding is considered in terms of nucleation and propagation of [001] screw dislocations with Burgers vector c/2 due to migration of 180 degrees chain twist defects. The stress-induced twist motion within the dislocation cores is modeled as an Eyring activated rate process. This gives an inelastic contribution to the dislocation core energy depending on the stem length and the strain rate and results ill improved predictions of the crystal plasticity approach. The model is compared to available experimental data as well as to the predictions of the modified crystal plasticity approach proposed by Brooks and Mukhtar. (c) 2005 Elsevier Ltd. All rights reserved.