We performed 560 minimally invasive unicompartmental knee arthroplasties in patients with osteoarthritis using the Oxford phase 3 prosthesis since January 2002. For the prospective analysis, clinical and radiographic assessments were carried out preoperatively and regularly followed after surgery. Ninety-five cases were followed at 3, 6, 12, 24, and 36 months postoperatively. In this prospective study, the early postoperative results of minimally invasive Oxford phase 3 unicompartmental knee arthroplasty were satisfactory for improvement of knee score and function score of the American Knee Society (AKS) score, and recovery of knee motion. The AKS knee score and function score continued to improve for 3 years postoperatively.