The functional knee hrace has been used to provide or to assist knee stahility in the anterior cruciate ligament (ACL) deficient knee, Nowadays, it is more indicated in the postoperative care of ACL injury to protect the reconstructed or repaired ligament, but eontroversies still exist regarding the effectiveness and the selection of an appropriate model. For successful application, it must restrict the knee motion within the range of certain arc, especially hyperextension, during weight bearing together with convenience and comfort.. The six commercial functional knee braces, which were introduced in Korea, were ehecked to detect the angle discrepancy between angle of brace and knee joint motion. The purpose of this study was to guide correct application of the functional knee brace in preventing hyperextension of the knee joint. Each brace was grouped into small, medium and large by size, and fixed with the arc of motion 30 to 90. The brace was applied to 3 healthy volunteers whose size was just fit mth the brace size small, rnedium and large, respectively. True lateral X-ray of the knee joint was taken in active flexion and extension state under the fluoroscopic control and measured the lateral intershaft angle of femur and tibia. Average angle discrepancy between the brace and the intershaft ang]e of femur and tibia in active extension was 22 and 3 in acTive flexion. In active extension at 30, angle discrepancy of Denzo brace was the large:st recording 26, that of Sentry hrace was the smallest recording 17. In active flexion at 90', angle discrcyancy of Sentry hrace was the largest recording 13, that of ACL brace (custom made modified Sentry brace) was the smallest recording 2. There was no statistically significant difference of the angle discrepancy among each size of brace. This study suggests that, at least 20 of angle discrepancy must be considered in appling the knee brace for prevention of hyperextension of the knee joint and careful selection of the brace, which may be affected by the design of hrace and the soft tissue compliance, is mandatory.