Objective To make a comparison for the change of maximum tensile intensity and stiffness of a whole implant that is placed into bone tunnel with various lengths tendon, by using beagle dog’s autogenous flexor tendons to reconstruct anterior cruciate l igament (ACL). Methods Sixty male beagle dogs were included in the experiment (weighting 13-16 kg). Three dogs were used for intact flexor tendon of both knees (normal control group), 3 dogs for the intact ACL andfemur-graft-tibia complex (auto control group) and 54 dogs (108 knees) for models of reconstructed ACL (6 experimentalgroups according to different lengths of tendon: 5, 9, 13, 17, 21 and 25 mm in the bone tunnel). The tensile intensity and stiffness were measured after 45, 90 and 180 days separately after operation. Results In the normal control group, the maximum tensile intensity of the intact flexor tendon was (564.15 ± 36.18) N, the stiffness was (59.89 ± 4.28) N/ mm. In the auto control group, the maximum tensile intensity of the intact ACL was (684.75 ± 48.10) N, the stiffness was (74.34 ± 6.99) N/ mm, all ruptured through the intra-articular portion of the graft. The maximum tensile intensity of femur-graft-tibia complex in the auto control group was (301.92 ± 15.04) N, the stiffness was (31.35 ± 1.97) N/mm. After 45 days of operation, all failure occurred at the tibial or femoral insertion site. After 90 days of operation, 24 of the breakpoints were scattered in tendon-bone junction, 12 (3 in 17 mm group, 5 in 21 mm group, 4 in 25 mm group) ruptured through the intra-articular portion. After 180 days of the operation, all breakpoints were distributed inside joint of the implant. The maximum tensile intensity and the stiffness were ber in 17, 21 and 25 mm groups than in 5, 9 and 13 mm groups after operation (P lt; 0.05). Conclusion Tendon with 17 mm length, which will be implanted into bone tunnel, is an appl icable index, in reconstruction of ACL by autogenous tendons.