A complex number mode analysis approach is proposed for vibration reducing of structural flexible redundant manipulators by utilizing self-motion. In the proposed approach, the self-motion is evaluated to nullify the modal exciting-force of flexural motion, and the approach can be freely used when the degree of freedom of flexural motion is much greater than the available degree of reundancy. The availability and effectiveness of the proposed approach are demonstrated through numerical simulation with a four-link spatial robotic manipulator possessing an end flexible link.