The synergistic effects of boron nitride (BN) powder and die on the rheology and processability of metallocene-catalyzed low density polyethylene (mLDPE) were investigated. The processability in the extrusion process is closely related to the interfacial properties between the polymer melts and the die wall. BN powder was added to mLDPE to reduce the friction coefficient and surface energy. Adding 0.5 wt% BN powder to mLDPE was very effective in improving the processability and the extrudate appearance. To study the effect of die surface property, three different dies were applied in capillary extrusion. One was a conventional tungsten carbide (TC) die, and the others were a hot-pressed BN (hpBN) die and a hot-pressed BN composite (hpBNC) die. The applications of these BN dies were quite effective in delaying surface melt fracture (sharkskin) and postponing gross melt fracture to a higher shear rate compared to the TC die. These improvements result from the fact that BN dies reduce the wall shear stress significantly and promote slip. The synergistic effect of processability could be obtained when both BN powder and hpBN die were used together.