【Abstract】 Objective To investigate corresponding relation between structure change of the femoral head with“crescent sign” and stress exerted on the avascular necrosis of femoral head, to explore the mechanism of the “crescent sign” formation. Methods From March 1998 to April 2003, the femoral heads of 18 hips in 16 cases having osteonecrosis and “crescent sign” in X-ray film before total hi p arthroplasty, were collected. General and coronal section plane morphology of the femoral heads were observed. The princi ple of effective stress and stress concentration theory were used to explain the phenomena and structure changes in osteonecrosis of the femoral head. Results Cancellous bone existed as a threedimensional,interconnected network of trabeculae rods and plates, with 50%-90% of porosity and 20-30 mmHg bone marrow pressure. According to the definition of porous media, bones especially cancellous bone was a kind of sol id and l iquid two phases porous media. Cross-sectional structure changes in the junction between subchondral plate and cancellous were the place where stress concentrated. The principle of effective stress and stress concentration theory could explain the phenomena and their relationship that occurred in avascular necrosis of the femoral head. Conclusion The “crescent sign” starts in an area of very focal resorption in the subchondral plate laterally and peripherally. The focal resorption in the subchondral plate breaks the continuity of subchondral plate and causes stress concentration in the resorption region. The concentrated stress accumulates in the junction between subchondral plate and unrepaired necrotic cancellous bone brings on the fracture right below the subchondral plate. The focal resorption of the subchondral plate also provides a pathway for the pore water in the unrepaired necrotic bone skeleton to outflow, therefore cause effective stress increase and unrepaired necrotic bone skeleton be compacted by increased effective stress appl ied on unrepaired necrotic cancellous bone skeleton, and results in the volume decrease of unrepaired necrotic cancellous bone and the formation of cavum below the subchondral plate. The cavum shows “crescent sign” in the X-ray film.