Abstract: The design of an observer-based optimal guaranteed cost controller for a class of singular systems with state-delay and parameter uncertainties was presented. The uncertainties are assumed to be time-varying and norm-bounded. The sufficient conditions enabling a singular system with observer-based guaranteed cost controller to stabilize its linear state feedback is obtained by using a new method in combination with the convex optimization theory. The obtained control law that the descriptor time-delay systems is robust stabilizable and the quadratic guaranteed cost function value is optimal in the sense of minimizing the upper bound of the cost function as proposed. It is also shown that these conditions are equivalent to the solvability of some linear matrix inequalities. A numerical example was provided to prove the validity of the proposed method.