Abstract: The optimal disturbance rejection problem for linear time-delay systems affected by sinusoidal disturbances is considered. The paper proposes a sensitivity approach to design of feedforward and feedback optimal control law. By introducing a sensitivity parameter and expanding power series around it, the optimal control problem is reduced to a series of two-point boundary value problems without time-advance or time-delay terms. By intercepting finite sum of the optimal control series, a suboptimal control law for the system is obtained. Simulations show that the algorithm presented in this paper is more efficient to damp externalsinusoidal disturbances than the classical feedback optimal control law.