The basic concept of the Dynesys dynamic stabilization system is to reduce the stiffness of the rigid fixation system to allow limited motion at the implanted level and to prevent stress concentrated at adjacent levels. However, the magnitude of cord pretension may change the stiffness of the Dynesys system and result in diverse clinical outcome, but the effects of Dynesys cord pretension remain unclear. Displacement-controlled finite element analysis was used to evaluate the biomechanical behavior of the lumbar spine after insertion of Dynesys with three different cord pretensions. For the implanted level, increasing the cord pretension from 100 to 300N resulted in an increase in stiffness in flexion from 19.0 to 64.5Nm/degree, and an increase in the high stress region of pedicle screw in flexion and lateral bending. For the adjacent levels, varying the cord pretension from 100 to 300N only has a minor influence on range of motion (ROM) of 6%. This study found that alteration of cord pretension affects the ROM within construct but not significantly to the adjacent segment. In addition, use of a 300N cord pretension causes a much higher stiffness at the implanted level when compared with the intact lumbar spine.