BACKGROUND: Healing of Critical-Sized Defects (CSDs) is one of the major challenges facing orthopedic surgeons. To assess the bone healing process usually plain radiography is used. Serial radiography results in certain side effects and recent findings are indicating the early detection of bone healing via ultrasonography. Objectives: The purpose of current study is to compare the diagnostic accuracy of radiography and ultrasonography in healing process of radial CSDs in rabbit. Methods: Sixteen New Zealand White Rabbits were used in this study. The radial CSDs of 15 mm size were created in a routine surgical procedure described previously and the two diagnostic tools were compared 12 weeks post-surgery. In case of obtaining different results from radiology and ultrasonography, to determine which diagnostic imaging method is of more accuracy, the histopathologic results of samples from the same day were used. Results: In 15 cases of 16, ultrasonography findings were confirmed by radiography and only in one case they were in contradiction, in which radiographs showed an Atrophic Nonunion while Ultrasound detected an outstanding filled defect with a reverberation artifact underneath. In histopathology, the defect was filled with an osteoid matrix of high cellularity and calcification was obvious in some regions, confirming the ultrasound results. Conclusions: This study indicates that in diagnostic imaging of CSDs, the ultrasonography and radiography are usually consonant and even ultrasound is more accurate than radiology. In addition, in case of detection of a reverberation artifact and lack of any healing-related finding in radiography, this artifact may be an indication of osteoid matrix formation.