The mechanical properties of composites consisting of high-density polyethylene (HDPE) and oil palm fibrous wastes-that is, empty fruit bunch (EFB)-have been investigated. Tensile modulus showed an increase, whereas tensile strength, elongation at break, and impact strength decreased with increasing filler loading. The strong tendency of EFB to exist in the form of fiber bundles and the poor filler-matrix interaction is believed to be responsible for the poor strength displayed by the composites. Attempts to improve these properties using two types of coupling agents, that is, 3-aminopropyltrimethoxysilane (S-APM) and 3-aminopropyltriethoxysilane (3-APM) and two types of compatibilizers, poly(propylene-acrylic acid) (PPAA) and poly(propylene-ethylene-acrylic acid), (PPEAA), are described. While almost all chemical treatments increased the stiffness of the composites, limited improvement has been observed in the case of tensile strength. This have been attributed to the presence of fiber bundles that remain intact even after several types of chemical treatment have been carried out. Thus, the role of EFB as reinforcing agent is not fully realized. Scanning electron microscopy (SEM) micrographs revealed that the main energy-absorbing mechanisms contributing towards toughness enhancement is through the fiber bundle pull-out process.