Objective To investigate the effect of the anterior cruciate l igament (ACL) rupture on the biomechanics of the lateral collateral l igament (LCL). Methods The specimens in this experiment were knee joints from 6 normal fresh adult male cadavers which was donated voluntarily, aged of 26-35 years with an average of 31.4 years. The knee joints were dissymmetry with 3 in left and right sides respectively. At first, all the 6 specimens lying on the self-made movement tooting, whose LCL straining were measured by strain gauges at different angles (0, 30, 60 and 90°) under axial loads of 400 N by e-testing machine for simulation of the normal knee joint force, were regarded as the intact ACL group. Then, the ACL in all 6 specimens were cut off completely as the deficiency group and did the same step. Results The strain of the LCL were as follows at 0, 30, 60 and 90°: (0.00 ± 1.63), (—17.2 ± 8.57), (—24.00 ± 4.80) and (26.50 ± 4.65) με in intact ACL group; (0.75 ± 8.22), (— 52.75 ± 3.33), (24.30 ± 4.99) and (26.30 ± 4.27) με in deficiency group. There was no significant difference at 0° and 90° flexion (P gt; 0.05), but significant difference at 30° and 60° flexion (P lt; 0.05) between the two groups. Conclusion The rupture of the ACL has significantly effect on the strain of the LCL which suffering abnormal load in the state at 30° and 60° flexion. At 30° flexion the relaxation of the LCL increased, which means the possibil ity of the injury of the LCL is rare; and at 60° flexion the LCL become very tense and thereby at the high risk of instabil ity.