This paper proposes a new observer gain tuning methodology to achieve a robust position control. The state observer is often used to estimate the state of the system. Generally, the observer gain is tuned by an evaluation function and the operator needs to arbitrarily select a weight function to obtain an optimal observer gain. This paper introduces a methodology for observer gain tuning using the stability margin theory. In addition, the state observer is combined with the Disturbance Observer (DOB) to achieve a robust position control. The validity of the proposed methodology was tested and confirmed by simulations and experimental study.