Hydroxyapatite/poly (ethylene adipate)-co-poly(ethylene terephthalate) biomaterials (HAp/PEA-co-PET) have been prepared by ring opening polymerization (ROP) of cyclic oligo(ethylene adipate)-co-oligo(ethylene terephthalate) (C-OEA-co-C-OET) in the porous hydroxyapatite (HAp) templates. The HAp powder was firstly synthesized from calcium hydroxide and phosphoric acid by co-precipitation method and then shaped to rectangular form. The porous HAp templates were obtained after firing at 1,100.C for 5 hours. The C-OEA and C-OET were synthesized by cyclodepolymerization of PEA and PET precursors, respectively, under high dilution condition using dibutyl tinoxide as a catalyst. The mixture of C-OEA and C-OET, having 1:1 molar ratio, was prepared and then refluxed for 7 days in order to obtain the co-cyclic of C-OEA-co-C-OET. The HAp/PEA-co-PET composites were prepared by soaking the porous HAp templates in the C-OEA-co-C-OET solution, and then in-situ ring-opening polymerization at 250.C for 24 hours under vacuum. The ring-opening polymerized PEA-co-PET was obtained as a successive polymer layer coating on the porous surfaces of HAp templates. The ROP-PEA-co-PET content in the composite was about 24 wt% with the number average molecular weight ( n M ) and weight average molecular weight ( W M ) of 1740 and 4550 g/mol, respectively. Compressive strength and modulus of the HAp/PEA-co-PET composites were about 29 and 246 MPa, respectively. These mechanical properties were higher than those of the initial HAp template and natural cancellous bone. In vitro bioactivity of the HAp/PEA-co-PET composites was studied by soaking in flowing simulated body fluid (SBF), having pH and concentrations of inorganic ions similar to human blood plasma. The formation of hydroxyapatite nanocrystal was observed on the composite surfaces through the consumption of calcium (Ca) and phosphorus (P) from the SBF solution, indicating the bioactivity of these HAp/PEA-co-PET composites. The elemental mappings of the HAp/PEA-co-PET surface after prolonged soaking for 28 days were mainly composed of Ca and P, having the Ca/P molar ratio of 1.67 similar to the theoretical value of hydroxyapatite. These results suggested that these HAp/PEA-co-PET composites are possible for use as an implant material for medical applications.