This note presents a new analysis method to design an observer-based dynamic surface controller (ODSC) for a class of nonlinear systems. While the separation principle from linear system theory does not generally hold for nonlinear systems, a separation principle for the ODSC systems will be shown for a class of nonlinear systems, thus enabling the independent design of the observer and DSC. Furthermore, a convex optimization problem to test quadratic stability of ODSC will be formulated later.