To investigate the suitability of calcium phosphate nanoparticles (CaP-NPs) as bone graft substitute in an experimental critical-size femoral bone defect (0.5 cm), six apparent clinically healthy mongrel male dogs of, average age 2 years, average weight 10-20 kg.b.w.t were randomly allocated into two equal experimental groups (3dog each).Control group without any implants, and treated group received CaP-NPs implant. Firstly, transverse femoral diaphyseal osteoctomy was made by Gegli saw. Then diaphyseal critical-size femoral bone defect (0.5 cm) was fixed by 3.5mm dynamic compression plate with 6 holes. Finally, the defect was filled with CaP-NPs. The observation period extend to 90 days post operation. During which the operated animals were evaluated clinically, radiologically and histopathologically. And also biochemical evaluation and computed tomography examination were also conducted to confirm the result. . Clinically, CaP-NPs group was superior to control in full weight bearing, soft tissue healing and resolution of inflammation at 10-15 days post-operation. Radiologically, the defect was completely filled with new bone formation after 10 weeks in CaP-NPs group. The control group showed a relatively slow healing process, and the union was complete after 13 weeks. Biochemical analysis showed a significant elevation of ALP activity, Ca, and Ph in CaP-NPs group. Computed tomography and histopathological examination at 13 weeks revealed better bone healing through marked bridging callus formation in CaP-NPs group than control one. The obtained result revealed CaP-NPs is a promising bone grafting material for treatment of bone defects due to its biocompatibility, osteoconductive and osteoinductive properties.