Objective To explore the best flexion angle of the transplantation tendon for fixing joint in simultaneously reconstructing of the anterior cruciate l igament (ACL) and posterior cruciate l igament (PCL) using semitendinosus tendon as autologous graft. Methods Twenty-four clean level New Zealand White rabbits [(aged 6-8 months, male or female, and weighing (2.5 ± 0.2) kg] were selected and divided randomly into 3 groups (n=8) according to fixation angle of the reconstructed l igaments. The bilateral semitendinosus tendons of hind legs were used to reconstruct the PCL and ACL of right hind leg, and the reconstructed l igaments were fixed at knee flexion angles of 90° (group A), 60° (group B), and 30° (group A). The rabbit general situation was observed after operation, and the specimens of the knee joints (including 10 cmdistal end and 10 cm proximal end) were harvested for testing extension and flexion, displacement, and internal and external rotation at 3 months after operation. Results All the rabbits survived to the end of experiment. There was no significant difference in maximal displacements of ACL and PCL among 3 groups (P gt; 0.05). The anterior and posterior displacements of shift in 3 groups were less than 1 mm, suggesting good stabil ity. The anterior displacement and the posterior displacement at 30° flexion and 90° flexion in group A were significantly larger than those in group C (P lt; 0.05). There were significant differences in internal rotation angle and external rotation angle between group A and group C (P lt; 0.05), and there was no significant difference among other groups (P gt; 0.05). Conclusion When simultaneously reconstructing ACL and PCL, the knee flexion angle of 60° for fixing the reconstructed l igaments can achieve the best effect.