PURPOSE The purpose of this study was to investigate the effects of muscle deficit of knee extension angle 30° on knee functional scores following anterior cruciate ligament reconstruction. METHODS The patients who had less than 20% of peak torque value following reconstructive ACL operation were participated in this study. In addition, they were divided into a group with knee extension angle 30° muscle deficit less than 20% (Control group; CON, n = 15) and into a group with knee extension angle 30° muscle deficit more than 20% (Muscle deficit group; MDG, n = 15). Knee extension angle 30° muscle deficit, IKDC and Lysholm scores were measured in knee extension angle 30°. RESULTS Knee extension angle 30° muscle deficit was significantly higher in the MDG compared with CON (p<.001). IKDC and Lysholm scores were significantly lower in the MDG compared with CON (p<.001). CONCLUSIONS This study demonstrated that a larger knee extension angle 30° muscle deficit following reconstructive ACL operation reduces IKDC and Lysholm scores. Therefore, it is recommended that knee extension angle 30° muscle deficit should be considered for rehabilitation and return to sports after ACL reconstruction.