Background: Anterior cruciate ligament (ACL) injury is the most common sports injury in both athletes and nonathletes; it can cause disability if not treated correctly. In cases with minor injuries, conservative treatments suffice. But, in cases with ACL tear, surgery by different methods and autografts are indicated. The most prevalent method for ACL reconstruction is the use of hamstring tendon autograft; this requires tendon removal and results in subsequent weakness in patient’s knee flexion strength which can cause dissatisfaction. Objectives: In this study we evaluate a common procedure used for treating ACL injuries. Patients and Methods: This study was performed at a hospital in Tehran on 30 patients with ACL tears. Patients’ knee flexion strengths before and 2, 4, 6, and 12 months after reconstruction were measured separately at 20, 45, 90, and 110˚ knee flexion angles, and their means were analyzed using paired t-test. Results: In this study, knee flexion strength decreased after ACL reconstruction. The greatest decrease in knee flexion strength was observed at 90 and 110˚ knee flexion angles. Conclusions: Some previous studies have confirmed reduced knee flexion strength following ACL reconstruction at high knee flexion angles. However, some others have denied it. The present study confirmed the reduction in knee flexion strength one year after ACL reconstruction at 90 and 110˚ flexion angles (P = 0.000). Furthermore, the need for physiotherapy, as a process for rehabilitating these patients was also confirmed