Abstract: Robust partial-state observer design was investigated for a class of nonlinear systems with disturbance. A novel H_∞ state observer was designed through which the partial-state of nonlinear systems with L₂(i.e. square integrable) disturbance can be reconstructed.First, the restricted conditions and the detailed expressions were given for the partial-state observer design matrices. Second, the L₂ disturbance attenuation problem was investigated for observer error by analyzing observer error dynamics, and sufficient conditions were given for the existence of the H_∞ partial-state observer based on the LMI method. Theoretical analysis shows that this designed observer can realize the asymptotic reconstruction for the states to be estimated when no disturbance, and the observer error is attenuation to the L₂ disturbance when disturbance exists. Finally,a simulation example was given to illustrate the correctness of the theoretical results of this paper.

Key words: nonlinear systems, robust partial-state observer, L₂disturbance, LMI method