Eigenstructure assignment is a typical method with the capability of consideration of the time-domain specifications in designing a linear control system. In this paper, we propose a new method for eigenstructure to achieve desired eigenvectors more precisely than with the conventional method. In the proposed method, the conventional eigenstructure assignment problem is interpreted as a constrained optimization one, and it converted into an unconstrained optimization problem to deal with the problem easily. Numerical examples are presented to illustrate the proposed flexible eigenstructure assignment method.