Generally, the flexible robot system can be modeled as the two-mass system which consists of a motor and load connected by a spring coefficient. In order to achieve accurate tracking objectives, the correct of parameter identification in the model based controller is necessary. The objective of this paper is the parameter identification of a spring coefficient of flexible robot system. The multi-encoder based disturbance observer (MEDOB) and load side disturbance observer (LDOB) are employed to identify a correct spring coefficient parameter of flexible robot system. The comparison of external force estimation between MEDOB and LDOB is used to find the correct value of spring coefficient. By using the proposed identification method, it is simple to identify the spring coefficient and easy to implement in the real flexible robot system. The effectiveness of the proposed identification method is verified by simulation and experimental results.