Wall slip is often seen as a source of flow enhancement and can play a significant role during certain melt instabilities in polymer processing. In the present investigation, the effect of organoclay on the wall slip of high density polyethylene (HDPE) was investigated with the aid of particle image velocimetry (Ply). The study showed that organoclay did not cause significant wall slip during low shear testing in a parallel plate rheometer. However, NV measurements showed that organoclay induced more wall slip during continuous extrusion of HDPE. Also, reduction in the extrusion pressure and elimination/postponement of melt instabilities in HDPE during continuous extrusion were observed. There was good agreement between the flow curves obtained from PIV and rheological measurements. In the presence of high shear flow, organoclay was suggested to align in the flow direction and migrate towards the die wall. The alignment and migration affect the bulk rheological properties (increase in shear thinning) and surface properties (increase in wall slip) of HDPE. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.