A systematic phase design method is developed for delay-based output feedback control of flexible mechanical systems. By this method, closed-loop stability and system performance are guaranteed. The design method is a phase planning scheme that makes use of the phase characteristics of different delay and delay-free control units. Together with a minimum destabilizing gain condition, the phase design method yields a sufficient condition of stability for any delay-based output feedback systems. Simulations show that the delay-based output feedback control can gain a wider closed-loop bandwidth than the conventional full state feedback.