A hydroxyapatite/poly(ethylene-co-acrylic acid) (HAp/EAA) nanocomposite has been synthesized by a solution-based method. p-Aminophenyl phosphonic acid has been used as a coupling agent in order to enhance the bonding between HAp and EAA, and hence to improve the mechanical properties of the composite. XRD study has indicated the development of compressive and tensile stresses in a nanocomposite due to thermal expansion mismatch between nano-hydroxyapatite (n-HAp) and EAA. Fourier-transform infrared spectrometry (FTIR) and thermal analysis have shown the presence of strong interfacial bonding between n-HAp and EAA. The surface roughness and the homogeneous dispersion of nanoparticles have been observed by field emission scanning electron microscopy (FESEM). A comparison of mechanical properties between phosphonic acid treated (cn-HAp/EAA) and untreated (un-HAp/EAA) nanocomposites has been made. The use of a phosphonic acid coupling agent promotes the uniform dispersion of n-HAp in the polymer matrix with a strong nanoparticle-polymer interfacial bonding, which provides a means of preparing a HAp/polymer nanocomposite for implant applications.