Abstract: The observability for bearing-only target motion analysis is a very important problem. The method of Forge & Gavish put forward a Nth-order dynamics target observability condition from angle measurements. The F-G method can not be used when the target motion equation can not be written in the form of polynomial. Using the nonlinear system theory we put forward a new observability method for bearing-only target motion analysis. This method is more general than the previous ones.