Objective:Discussing the worth of MRI used in the diagnosis of osteochondritis dissecans.Methods:Collecting 17 caes of Osteochondritis dissecans proved by arthroscopy and surgery,retrospective analuping thire image materials about MRI.Results:14 of the cases in the knee medial or cateral condyle,the other 3 cases in talus trochlear.The manifestation about the course of disease diagnosed by MRI Just as same as the arthroscopy and pathology discovered in operation.Conclusion:THE MRI of the osteochondritis dissecans has a specific manifestation.MR is the most sensitive and effective image way for diagnosing the Osteochondritis dissecans.