Objective: This experimental study was performed to explore the intraosseous microvascular alterations in the irradiated mandible and to increase understanding of the pathogenesis of osteoradionecrosis. Methods: One hundred twenty-eight guinea pigs were grouped according to different radiation sources and dosages. Fractioned radiation was delivered to the right mandibles. Dental extractions were conducted at 1, 3, and 5 months respectively, after radiation. One month later, the animals were examined by means of gross observation, histopathology, and microvascular corrosion castings. Results: Histologic evaluation showed bone absorption within 2 months following radiation. Four months later, the number of osteocytes decreased and pyknosis and empty lacunae were commonly seen. The casting specimens revealed uner scanning electron microscopy that capillary disconnection and obliteration with subsequent vein shrinkage aggravated over time. In the nonhealing dental extraction site, a large-scale vascular network defect presented with focal capillary proliferation around. Conclusions: The capillary is the part most vulnerable to the damage caused by radiation in the vascular system of the mandible. On the basis of ‘hypvascular, hypoxic, hypocellular’ structure, local microcirculation failure induced by the trauma-repairing process leads to occurrence of osteoradionecrosis of jaws.