Objective To investigate the value of MRI in grading diagnosis of cartilage injury of knee joint. Methods The MRI and clinical date of 56 patients with cartilage injury of knee joint confirmed by arthroscopy or operation in our hospital were collected. The all cartilage injury was graded and diagnosed by the Outerbridge Classification Criterion,and we compared the results of MRI imaging with those by arthroscopy or operation. Results Among all 336 joint surfaces of 56 cases in this group,Ⅰ、Ⅱ、Ⅲ、Ⅳ injury diagnosed by MRI imaging was 26、28、58、43; while the results by arthroscopy was 27、27、58、43.Taking the classification diagnostic results of arthroscopy as standards,the sensitivity、specificity、accuracy rate of MRI imaging for cartilage injury of knee joint was 97.45％、98.32％、97.92％，negative predictive value was 97.78％,and positive predictive value of Ⅰ、Ⅱ、Ⅲ、Ⅳ injury was 69.23％、75％、100％、 100％.Compared with the results by arthroscopy,the MRI classification diagnostic results of cartilage injury of knee joint have superb consistency（Kappa=0.909）,besides, The difference was statistically significant(P＜0.05). Conlusion MRI can objectively and reliably display the location and degree of the cartilage injury of knee joint, provide clinic with the reliable evidence, grade and diagnose cartilage injury of knee joint before operation and be worth popularizing and applying widely.