Abstract: The robustness with a class of reduced-order state observers based control systems Is discussed in this paper.The results of〔1〕are extended to a class of reduced-roder case.A scheme of pole-assignment of the reduced-order observers is proposed.Anexample and simulation results are given.It turns out that when the pole-assignment of a reduced-order observer is carried out,the fast response (and the noise restriction)and the robustness of the system must be considered simultaneously.