Background: Osteoarthritis (OA) is a leading cause of disability and frequent visits to primary care practitioners for those with the condition in comparison to those without. OA affects the knee more often than any other joint of the human body. Platelet-rich plasma (PRP) which is also known as autologous conditioned plasma is a concentrate of platelet-rich plasma protein derived from whole blood, centrifuged to remove red blood cells. Now it is used in treating OA also. Aim of the study: The aim of this study was to evaluate Effectiveness of Platelet-Rich Plasma in the Treatment of Knee Osteoarthritis with WOMAC Score. Materials and methods: This prospective observational study was conducted in the Department of Orthopaedic Surgery, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh from 15th July 2017 to 30th June 2019. In total 120 patients more than 40 years of age, diagnosed with grade I and II arthritis were included in this study as the study population. To all the participants three PRP injections were injected in the knees at 4-week interval. Patients were followed up and were assessed for pain and physical activity associated with arthritis using the Western Ontario and McMaster University Arthritis Index (WOMAC) score at the time of induction in the study and at 6 months interval. Results: The mean WOMAC score before the starting of treatment was 82.94 (±5.12) and after 6 months it was reduced to 37.87 (±4.73), which was statistically significant (p= 0.0001). WOMAC score was improved more in the subgroup with patients having symptoms less than 2 years (p=0.0254). On the other hand, in case of subgroups with Kellgren-Lawrence grade 1 or 2 the p value was 0.0730 and there was not any significant difference in the WOMAC score at 6 months among males or females (p=0.1511). Conclusions: The proper sequential use of PRP injection for the treatment of osteoarthritis of knee-joint proved to be effective in our study. The findings of this study may be helpful in the treatment arena of osteoarthritis and in further similar researches.