The aim of this study was to analyze the process of restoration of muscle power following arthroscopic primary repair of ACL with manual test and Cybex test for one year postoperatively. Arthroscopic primary repair of the ACL was performed in 24 fresh tear. Postoperatively a limited motion brace was applied. Full weight bearing was allowed at 10 weeks postoperatively. In all cases mid-thigh circumference at 7.5 cm proximal to the upper pole of patella was measured. Manual muscle power test and Cybex test were also performed and the differences between the normal and the affected legs were recorded. The manual test and measurement of mid-thigh circumference were performed at 6 weeks, 12 weeks, 6 months and 1 year postoperatively. All cases showed 'normal' muscle power at 6 weeks after surgery in manual test. The mean difference of mid-thigh circumference was 4.5 cm(range 2.0-6.0 cm) at 6 weeks, 3.6 cm(range 1.0-6.5 cm) at 12 weeks, 2.9 cm(range 1.8-4.8 cm) at 6 months and 0.9 cm(0.5-2.5 cm) at 1 year after surgery. The peak torque deficit measured by Cybex test showed 50-82%(average 65.8%) in extensor and 24-96 %(average 60.6 %) in flexor at 12 weeks, 21-60 %(average 41.4 %) in extensor and 2-50 %(average 32.4 %) in flexor at 6 months, 1-23 %(average 13.1 %) in extensor and 1-19 %(average 11.4 %) in flexor at 1 year after surgery. From these results it would be suggested that the peak torque of muscles was not normal by Cybex test although the muscle power was restored clinically by 1 year following surgery.