Abstract: The design methods of full-order observers for affine nonlinear systems are discussed and a kind of design method is developed.The design method is based on the positive definite solution of a differential Riccati equation,with which the time-varying gain matrix of the observer is determined.The existence of the observer is proved by a way of coordinate transformation based on Lyapunov theory.The simulation results of an actual model show the practicality and validity of the proposed observer.