Purpose: The purpose of this study was to compare the morphologic change of the posterior cruclate ligament (PCL) on magnetic resonance imaging (MRI) in chronic anterior cruciate ligament (ACL) tear before and After reconstruction. Materials and Methods: On the MRI image after ACL reconstruction, a base line (BL) was drewn drawn between the femoral and tibial in sertion sites of the PCL. Point A was established as the vertically farthest point from BL. Line I and 2 were drawn respectively between point A and the insertions of PCL. The angles between BL and Linel and 2 were define as angle a and angle b. Correlations between the preoperative anterior knee instability assessed with the KT 1000 arthrometer and the parameters were evaluated. Results: The average angle a was 52.3°±7.1° preoperatively and 30.1°±3.4° postoperatively, and the average angle b was 28.95°±4.62° preoperatively and 22°±2.9° postoperatively (p<0.05). The preoperative manual maximal side-to-side difference was 7.2±2.3 mm, which had no significant correlation with angle b. Comclusions: The preoperative buckling of PCL in the case of chronic ACL injury disappeared after ACL recon. The degree of PCL buckling had no correlation with the degree of preoperative anterior instability.