The aim of this work was to study the role of effective parameters in the morphology evolution of polymer blends in a twin-screw extruder with an emphasis on the effects of the melt viscoelastic properties of the blend components, screw speed, and flow field changes along the extruder. Two sets of blend samples based on polypropylene/polyamide 6 with the same composition (90/10) and polypropylene matrix but with two polyamide 6 grades differing in their viscoelastic properties were considered. The effect of the compatibilizer (maleic anhydride grafted polypropylene) was also investigated. The morphology of the blend samples were studied with scanning electron microscopy, and the melt linear viscoelastic properties of the samples were measured with a rheometric mechanical spectrometer. The melt-blending processes were carried out in a modular twin-screw extruder. The trend of the polyamide dispersed particle size changing with increasing screw speed was found to be different for the two types of blends. Considering of the role of the compatibilizer in coalescence, this could mainly be attributed to the different melt viscoelastic properties of the blend constituents. Similar explanations were given for the morphological changes that occurred along the screw. It was demonstrated that the viscosity ratio of the blend components is not necessarily the only material-dependent parameter that affects the dispersed particle size: the viscoelastic properties (particularly the flow-induced elasticity ratio) of the blend components can also play a significant role in determining the particle size distribution. (C) 2008 Wiley Periodicals, Inc.