The Luenberger observer design method is extended by a furtber interesting consideraron. This paper addresses the design method of robust observers for linear systems with uncertainty and a class of nonlinear systems by applying the concept of system roubstness degree proposed by the authors.4) The uncertainty in linear systems and nonlinear distortion in nonlinear systems may lead the observer to be unstuble. The Lueuberger observer design appruach combined with the system robustness degree will construct a robust asymptotic observer. Reduced-order robust observer design is also studied. Finally, a practical example of nonlinear robust observer design for a biochemical reactor is illustrated to show the validity of the proposed method. © 1989, The Society of Chemical Engineers, Japan. All rights reserved.