Bioresorbable composites consist of micro- and nano-sized α-tricalcium phosphate (α-TCP) particles in a poly(D,L-lactic-co-glycolic acid) (PLGA) matrix were compared for their ability to form bone-like apatite in simulated body fluid (SBF). Due to the uniform distribution of α-TCP nanoparticles over the nanocomposite surface, enhanced apatite formation was observed. Compared with microcomposite with the same α-TCP load, this enhanced apatite formation was achieved through faster and more uniform apatite nucleation. In the first stage of apatite formation, the presence of abundant high energy boundaries between α-TCP nanoparticles and PLGA matrix in nanocomposite provided a large number of suitable sites for calcium phosphate (CaP) nucleation. A homogenous distribution fo CaP nuclei formed after days. The resulting apatite crystals grew to form a flake-like apatite layer. In contrast, CaP nucleation was only observed on the micrometre-size α-TCP particles in the microcomposite over the same period. After 14 days, a dense, flake-like apatite was visible covering the surface of nanocomposite, whilst this suface layer was formed only on α-TCP particles of the microcomposite.