Abstract: The toughening and reinforcement mechanism of HA ( hydroxyapatite ) /HDPE ( high density polyethylene ) composite was investigated. The results show that the co-operative actions of multi-scale is the main role to enhance the mechanical properties. On the nano-scale, the homogeneous dispersion of HA and high interfacial strength between HA and HDPE leads to the high crystalline, litter crystal size and the formation of oriented extended-chain crystal structure. This will result in the HDPE deformation and HA deadhesion during the fracture process, which can pin the development of microcracks, expend large energies and prohibit the formation of large crack. On the micro-scale, the formation of oriented HA/HDPE fibers can greatly reduce the activity volume and increase the fracture energy, which leads to the high st rength. On the other hand, the composite fiber can expend much more energy by fracture, pullout and crackbowing, which enhances the strength and toughness to a great extent.