Background: Osteoarthritis (OA) is one of the crucial musculoskeletal disorders that are characterized by the imbalanced homeostasis and destruction of the articular cartilage. Objective: This study aimed to evaluate the effect of platelet-rich plasma (PRP) in treatment of knee osteoarthritis. Patients and Methods: This interventional study was carried out on 20 patients who were suffering from mild to moderate primary osteoarthritis of knee joint during the period from May 2012 to March 2013. They were diagnosed clinically, and by plain x- ray and Doppler musculoskeletal ultrasound. Results: There were 5 patients (25%) experienced slight pain at the site of injection lasted for one week and only one patient (5%) experienced marked pain. Two patients (10%) had skin discoloration in the form of bruising. No reported cases suffered from infection nor allergic reaction.  Conclusion: we concluded that PRP intra-articular injection is an effective method for treatment of knee OA. It is a safe & economic alternative method of treatment. Maximal improvement is obtained in patients with young age and short disease duration.