Abstract: A novel robust adaptive observer is designed for a class of nonlinear systems with mismatch time-varying uncertainties. First, the systems are transformed into observer-like canonical form by geometric methods. Secondly, the compensated term is introduced in the observer, which makes the transfer function between compensated errors and estimated error strictly positive real. As the design and the theoretical analysis are unified, unreasonable limitations to nonlinear dynamics, such as matching and Lipschitz conditions, are removed.