This study was conducted to compare the differences of the outcome and surgical technique for minimally invasive unicompartmental knee arthroplasty (UKA) in treatment of osteonecrosis versus osteoarthritis. Twenty-nine spontaneous osteonecrosis of the knee (SONK) cases were reviewed retrospectively. An equal number of patients with osteoarthritis (OA) performed in the same period were selected and matched with respect to age, preoperative range of motion and radiological grade of knee arthrosis. The mean follow up time were 44.14 +/- 14.05 and 44.45 +/- 14.45 months, respectively. The preoperative hospital for special surgery knee score and visual analogue score were significantly better in group OA than those of group SONK. However, the results were comparable in terms of postoperative pain, knee score, range of motion and axial alignment. From a technical point of view, the osteonecrosis stage and bone defect must be taken into account when using UKA for SONK.