In this work, phase separation in carbon nanofiber (CNF) composites with a blend of ultrahigh molecular weight polyethylene (UHMWPE)/high-density polyethylene (HDPE) was revealed, and its effects on tribological properties were investigated. Results from morphological analysis by optical and scanning electron microscopy indicated two distinct microstructures: a dispersed UHMWPE phase and a continuous microstructure containing HDPE and CNFs. The addition of CNFs into the UHMWPE/HDPE blend induced a decreased steady-state torque indicative of a decreased dissolution and improved processability. Because CNFs predominantly resided into the HDPE phase, neat HDPE, a HDPE/CNF composite, and neat UHMWPE samples were also prepared for comparison. Wear results, determined by a pin-on-disk apparatus, showed that both initial run-in and steady-state wear rates of the UHMWPE/HDPE/CNF nanocomposites were reduced with an increasing concentration of CNFs. The wear resistance of the UHMWPE/HDPE blend was more strongly influenced than neat HDPE by the addition of CNFs, which may have been affected by a reduced dissolution and improved interfacial interaction between the two phases. Results from this study suggested that HDPE may not be appropriate for processing UHMWPE composites, as CNFs reside in the HDPE phase, and HDPE diminishes the wear resistance of the material. POLYM. ENG. SCI., 50:613-623, 2010. (C) 2009 Society of Plastics Engineers