This paper deals with the parameter and state estimation problem for the Lipschitz nonlinear systems. First, a nonlinear adaptive observer for a class of Lipschitz nonlinear systems is designed, and based on LMI the stability conditions are derived. Then for a class of nonlinear system, the new sufficient conditions are derived, which ensure the convergence of the adaptive observer with perturbation on observer gain. The simulation result is presented to show the effectiveness of the proposed method.