Melt flow instability of LLDPE (Linear Low Density Polyethylene) and its composites with three kinds of carbon fillers with different dimensionalities, including multi-wall carbon nanotubes (MWCNTs), thermally reduced graphene (TRG) and carbon black (CB), was evaluated. A periodic oscillation in entrance pressure drop during the process of wall slip was observed. It was demonstrated that the dimensionalities of the fillers played a significant role in the rheological behaviors for LLDPE composites. The critical shear rate for the onset of the stick-slip transition increased when MWCNTs added but decreased with the addition of TRG. However, in terms of the LLDPE/CB composites, there was no obvious changes for this critical shear rate. Mechanisms for the effects of these fillers on the flow instabilities were proposed according to the molecular structure and the movement of polymer chains.