[Purpose] To compare two platelet-rich plasma kits with different platelet concentrations for treatment of knee osteoarthritis. [Subjects and Methods] Male and female patients with knee osteoarthritis who had confirmed diagnosis with X-ray and magnetic resonance imaging were included in this retrospective study. Eligible patients were divided into two groups: Group I, which received platelet-rich plasma kit I, and Group II, which received platelet-rich plasma kit II. Platelet concentrations of both kits were measured by manual counting. For each group, platelet-rich plasma kit was injected twice with a one-month interval between injections.