Silanation of hydroxyapatite and acrylic acid grafting of polyethylene were employed to improve bonding between hydroxyapatite and polyethylene. The structure and properties of chemically coupled hydroxyapatite reinforced high-density polyethylene (HA/HDPE) composites were subsequently investigated using various techniques. It was shown that there existed a silicon-containing interphase between the hydroxyapatite particle and the polyethylene matrix. The silane coupling agent used promoted chemical adhesion between hydroxyapatite particles and the polymer. The use of the silane coupling agent also facilitated the penetration of polymer into cavities in individual ceramic particles, which resulted in enhanced mechanical interlocking at the matrix-reinforcement interface. With a strong bond (both mechanical and chemical) being formed between hydroxyapatite and polyethylene, chemically coupled HA/HDPE composites possessed improved mechanical properties and fracture behaviour.