Pellegrini-Stieda lesion is described as ossification on the origin of medial collateral ligament. We present a case of end-stage knee osteoarthritis with the Pellegrini-Stieda lesion treated by unicompartmental knee arthroplasty (UKA). During the postoperative follow-up, an interesting change occurred such that the ossification lesion disappeared gradually and did not relapse. It is supposed that the disappearance was caused by UKA changing the abnormal biomechanics of the varus osteoarthritic knee.