Abstract: An observer-based adaptive neural-network H_∞ tracking control scheme is presented for a class of non-affine nonlinear systems with external disturbance and unavailable states.By using implicit function theorem,Taylor's formula and mean theorem,the form of the non-affine nonlinear systems is transformed into the form of affine nonlinear systems.The controller consists of an equivalent controller and H_∞ controller designed to attenuate the effect of external disturbance and approximation errors of the neural networks on tracking.The overall control scheme and the weight update laws based on Lyapunov stability theory can guarantee the system stability and asymptotic convergence of the tracking error to zero,and attenuate the effect of the disturbance on system to a prescribed level.Theoretical analysis and simulation results demonstrate the effectiveness of the approach.