Alumina (Al2O3) fiber/high density polyethylene (HDPE) composites were prepared by molding injection with or without compatibilizer, in which, maleic anhydride-grafted polyethylene (PE-g-MA) and acrylic acid-grafted polyethylene (PE-g-AA) were used as the compatibilizers. The thermal conductivities of the composites were anisotropic and the conductivities in the injection direction of the samples were higher than those in perpendicular direction of the injection. The anisotropic thermal conductivity for Al2O3/PE-g-AA/HDPE was the most obvious and this composite also gave the best mechanical performance. The SEM and DMA test revealed that PE-g-AA was more effective than PE-g-MA in improving the matrixfiller interaction. The high interfacial interaction was more favorable for the viscous flow-induced fiber orientation, which resulted in the largest anisotropic degree of thermal conductivity of the Al2O3/PE-g-AA/HDPE among the studied composite. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011