Patients with bilateral OA knee will be enrolled and randomized into intervention and control knee. One dose of PRP is applied in the intervention knee and normal saline in control side. Outcome measurements included visual analog scale (VAS)、Lequesne's severity index、Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)、balance test (Biodex) and isokinetic strengthening test at different follow-up frame (2nd week, 1st month, 3rd month and 6th month after treatment).