High tibial osteotomy (HTO) is a surgical procedure to re-align varus or valgus mal-alignment of the knee. Medial opening wedge HTO is a surgical treatment of choice for knee varus mal-alignment with many advantages over lateral closing wedge HTO. However, the opening of the medial wedge alters the plane of the lateral tibial plateau, especially when the correction angle is large, which may be the cause of early arthritic changes of the lateral compartment. Many recent studies reported the changes in medial posterior slope after medial opening wedge HTO, which affected the biomechanics of the knee. This report presents uni-condyle HTO, a new surgical technique that corrects alignment of medial tibial plateau in both coronal and sagittal plane without affecting the plane of lateral tibial plateau.