Abstract—Flexible joint single-link manipulator is a nonlinear system with many applications in industry such as car assembly plant, beer factory. However, this manipulator has many disadvantages including unknown parameters, external disturbances as well as holonomic constraint force. In order to overcome these challenges, many researchers employ the disturbance attenuation control scheme. It was hard to obtain the asymptotic stability in closed system due to the influence of disturbances. Moreover, the attraction region was also estimated exactly because of no knowledge of disturbance influence. This paper presents an external disturbance observer (DO) for flexible joint single-link manipulator. Moreover, the arbitrary small attraction region is obtained by using the suitable parameters. The main result of this paper is proposed based on theoretical analysis of differential equations without any traditional Lyapunov stability analysis. Furthermore, several explorations that depend on parameters are given out. Offline simulation results pointed out the high effectiveness of the proposed methods.