The purpose of this study was to examine the repairing ability of nano-hydroxyapatite (nano-HA) artificial bone with different pore sizes. Animal models of bone defects were created in both radii of 60 New Zealand white rabbits. The bone defects in A, B, and C groups were repaired with nano-HA artificial bone with three different pore sizes while those in group D were left unrepaired. The repairing ability of material was evaluated by gross observation, histopathological study, X-ray examination, scanning electron microscope (SEM), and biomechanical analysis in the fourth, eighth, and 12th weeks. Group B stimulated more bone formation than the other groups. Nano-HA artificial bone is capable of good bone formation and biocompatibility. The ability of bone formation of nano-HA artificial bone may be affected significantly by the pore size. The material with pore size in the range of 100 –250 μm has a greater ability to form bone.