The influence of a new processing additive (fine particles of boron nitride) on the processability of polyolefins and fluoropolymers in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and an extruder. Two metallocene polyethylenes and several Teflon(R) fluoropolymers were tested using these two pieces of equipment. The additive had a significant effect on the extrudate appearance of polyethylene and fluoropolymer particularly in the crosshead dies. It was found to eliminate surface melt fracture and to postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.005% to 0.5%). To explain the possible mechanism for the effect of the additive on the processability of the resins, rheological measurements using both parallel-plate and sliding-plate rheometers were carried out. The rheology of the resins did not seem to change significantly with the addition of boron nitride except for the low-shear-rate (low-frequency) range, where the behavior of the filled resin was found to be similar to that of a crosslinked polymer or a phase-separated entangled blend. Practical wire coating and tubing extrusion studies for these resins were also carried out.