In this paper, we explore the effects of hydroxyapatite microparticles and nanoparticles on the thermal and dynamic mechanical properties of injection moulded bioresorbable poly-L-lactide/hydroxyapatite composites intended for use in orthopaedics. The T-g of the nanocomposites were lower than those of the microcomposites. This was thought to be due to the larger surface area of well dispersed nanoparticles in the polymer matrix, leading to a larger interfacial area. The as-moulded composites were largely amorphous, however, during thermal analysis the polymer in the nanocomposites crystallized more and had lower cold crystallization temperature than that in the microcomposites since the nanoparticles acted as more effective nucleating agents. The storage moduli at 37 degrees C of the nanocomposites were higher than those of the microcomposites. The storage moduli of the composites approached the lower range of storage modulus for cortical bone and may prevent stress shielding during bone regeneration. Crown Copyright (c) 2011 Published by Elsevier Ltd. All rights reserved.