Objective To study the X-ray, CT, MR of the knee joint osteochondritis dissecans (OCD) imaging findings, the value in diagnosis of lesions and to compare the three methods of inspection. Methods Retrospective analysis of 38 cases (41)of CT, MR and X-ray findings that were confirmed OCD by knee arthroscopy and operation , and evaluated the imaging charateristics of osteochondritis dissecans. Results (1) the lesions occurred mostly in the medial femoral condyle (30 73.2%). (2) the X-ray image was manifested as ossicles peeled from articular surface , the density of them were higher, their edge were sharp, which were surrounded by bright line , and below them were bone bed to take them in with harden edge. (3) CT showed destruction of bone in the defect area as visible sequestrum, which did not fall off and can see part of them with one end of a bone at mostly free state remains connected to the bone cortex of the articular surface. (4) MR showed the performance of subchondralfocal focal small lesions, which appeared low signal on T1WI, low or high signal in the peripheral band around the lesion, the subchondral bone plate thinned or disappeared, part of articular cartilage fractured or vanished. (5) MR for lesion detection rate is higher. Conclusion X-ray, CT, MR expression of OCD is unique, MR can make a clear and definite diagnosis of it.