The effect of single (SWCNT) and multi- (MWCNT) walled carbon nanotubes on the melt flow instabilities of polyethylenes with different topologies was characterized by means of a novel capillary rheometer allowing in-situ measurements of the pressure fluctuations inside the die and by scanning electron microscopy (SEM) analysis. Our results show that carbon nanotubes modify the main characteristics of the spurt instability developed by the linear polyethylene. Furthermore, the sharkskin instability, developed in short chain branched polyethylene, is reduced at low amounts of MWCNT and SWCNT. Noteworthy, the critical shear rate for the on-set of the spurt and the sharkskin instabilities decreases in the nanocomposites due to the physical interactions between the polymer and the nanofiller. At high shear rates, the gross melt fracture instability is completely erased in the nanocomposites based on the linear polymer whereas in short chain branched polyethylene the amplitude of this bulk distortion is rather moderated. These changes were confirmed by on-line pressure measurements using the novel capillary rheometer set-up. Based on our results, it is concluded that carbon nanotubes drastically affect the non-linear molecular dynamic leading to polyethylene melt flow instabilities. (C) 2010 Elsevier Ltd. All rights reserved.