This article is concerned with the problem of observer-based passive control for descriptor systems with time delay. Sufficient conditions are first presented for the closed loop systems to be admissible and passive with dissipation ηin the case of that the time delay is known, and two kinds of methods are given to design the expected observer. Then, the case of that the time delay is unknown or uncertain is discussed. The observer-based control gains could be obtained from the solutions of linear matrix inequalities (LMIs). Simulation examples are given to show the effectiveness of the designed methods.