The biomechanical function of the iliolumbar ligament in the human lumbosacral junction was investigated by analyzing the three-dimensional movements of the whole lumbar and lumbo-sacral-ilium specimens. The experiment was repeated in the following three conditions: 1) intact iliolumbar ligament, 2) right iliolumbar ligament transected, and 3) bilateral iliolumbar ligaments transected. The representative values of the increased motions, compared with intact, after transection of the bilateral iliolumbar ligaments were 1.7 degrees (23%) in flexion, 1.1 degrees (20%) in extension, 0.3 degrees (18%) in axial rotation, and 1.2 degrees (29%) in lateral bending. The most restricted motion governed by the iliolumbar ligament in the lumbosacral junction was lateral bending. The bilateral iliolumbar ligament specimen could restrict flexion and extension of the lumbosacral junction, but the unilateral iliolumbar ligament preparation alone could not restrict these motions. The iliolumbar ligament also had the function of restricting the rotational movement of the lumbosacral junction.