Objective To evaluate the angle between the anterior talofibular ligament (ATFL) and posterior talofibular ligament (PTFL) in patients with mechanical ankle instability,so as to find out a new indirect diagnosis method for the chronic ATFL lesion.Methods One hundred subjects were included in this study,among which 50 patients with mechanical ankle instability were assigned to group A and 50 counterparts without it were selected into group B All patients were taken MR Images and the ATFL and PTFL angle at the axial plane was evaluated.The difference between two groups was evaluated.Results The ATFL and PTFL angle increased significantly in group A when compared to group B (82.1° ± 8.0° v.s,74.4° ± 5.9°,P＜0.01).The area under receiver operating characteristic curve (ROC)was 0.874 (P＜0.01),indicating great significance in diagnosing the chronic ATFL tear.Moreover,the angle was 79 degree,showing high sensitivity (0.88) and specificity (0.64) of diagnosing ATFL tear.Conclusion The ATFL and PTFL angle increases significantly in patients with the mechanical instability.Moreover,the possibility of ATFL tear increases when the ATFL and PTFL angle is greater than 79 degree.