Many types of bioactive materials are categorized as bone tissue substitutes for reconstruction and regeneration of bone defects, such as nano-hydroxyapatite. The objective of the present study was to radiologically evaluate the bone healing process in experimentally induced tibial defects in dogs treated with two different viscosity concentrations of nano-hydroxyapatite gel. Twelve adult, healthy Mongrel dogs were included. A critical size bone defect of 3–0.7 cm was induced surgically in the lateral border of the tibial bone of the right limb of all dogs enrolled in this study. The dogs were then categorized into two treatment groups: Group 1 (6 dogs): The defect was filled with prepared hydroxyapatite nanogel at a concentration of 33%, and Group 2 (6 dogs) hydroxyapatite nanogel at a concentration of 24% was used as a filling material. The healing process of the tibial defect and associated clinical and radiolographical findings were recorded in all studied groups at 30 and 60 days postoperatively. The results of the current study showed complete healing of the induced defect in the absence of any signs of pain or discomfort. Radiographically, there was an increase in radiographic density in the first group at 60 days. There is continuing healing in the late stage of the bone segment with the surrounding area and a crossing callus with cortical irregularities, denoting a chronic periosteal reaction and a good healing process. In the second group, the defect was completely filled with cortical thickening, which appears denser, denoting a periosteal reaction. In conclusion, using hydroxyapatite nanogel with high viscosity as a bone substitute contributed to progressing bone tissue regeneration with good callus formation and giving perfect mechanical support to defective bone.