The influence of a new processing additive (fine particles of boron nitride) on the rheology and processability of polyolefins is studied. The equipment used includes an Instron capillary rheometer equipped with capillary and special annular dies (Nokia Maillefer wire coating crosshead) and two rheometers, namely a parallel-plate and a sliding-plate rheometer. Several types of boron nitride powders, varying in average particle size and distribution and in morphology are tested at various concentration levels. The additive with the smallest average particle size and free of agglomeration was found to have the greatest influence on the processability (melt fracture performance) of the polyolefins tested. Specifically, it was found that boron nitride not only eliminates surface melt fracture but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values, depending on the additive concentration, surface energy, and morphology. A flow visualization technique was used to visualize the polymer flow at the entrance of a transparent capillary die in order to determine the mechanism by which boron nitride eliminates gross melt fracture.