To compare the morphological anatomy and abnormalities of the anterior talofibular ligament (ATFL) and calcaneofibular ligament (CFL) in unilateral chronic ankle instability (CAI).22 patients (men: women, 13:9; mean age, 28.95 ± 8.127 years) with unilateral CAI and 18 healthy volunteers (men: women, 9:9, mean age, 28.33 ± 3.678 years) were recruited. MRI scans were divided into Group 1 (22 injured ankles), Group 2 (22 contralateral uninjured ankles), and Group 3 (36 healthy volunteer ankles). The morphologic variables, MRI signal intensity (SI) values were evaluated.The ATFL proximal, intermediate, and distal sites and the CFL proximal and distal sites in Group 3 were narrower than those in Group 1 (P <0.05). Both ATFL and CFL in Group 1 were thicker than those in Group 3 (P <0.01). The proximal and intermediate sites of the ATFL and the proximal site of the CFL in Group 3 were narrower than those in Group 2 (P <0.01). The intermediate site of the ATFL and the proximal and distal sites of the CFL in Group 2 were thicker than those in Group 3 (P <0.01). The mean SI values of the ATFL in Group 1 were higher than those in Groups 2 and 3 (P <0.01). The ATFL and CFL SI values were higher in Group 2 than those in Group 3 (P <0.05).Both the injured and contralateral uninjured ankles had wider ATFL and CFL, more thickness, and higher SI values compared with those of healthy volunteer ankles.High-resolution three-dimensional MRI provides a potential tool assisting clinical decision on the treatment and rehabilitation therapy of patients with unilateral CAI.