The influence of polyethylene (PE) glycol (PEG), diatomite, and diatomite/PEG binary processing aid (BPA) on the rheological properties and the sharkskin melt fracture of three PEs was studied using a capillary rheometer. When diatomite or PEG is added to the PE matrix, they have little effect on the viscosity reduction of PEs, while the diatomite/PEG BPA shows a synergetic effect on the viscosity reduction of PEs. The incorporation of small amount of BPA was found to increase the shear-thinning behavior and decrease the melt viscosity significantly. Meanwhile, the critical apparent shear rate for the onset of sharkskin melt fracture of PEs is increased. The mechanism for BPA to improve the rheological properties 2005. (c) 2005 Society of Plastics Engineers.