This paper proposes new practical design tools for the robust motion control systems based on disturbance observer (DOB). Although DOB has long been used in several motion control applications, it has insufficient analysis and design tools. The paper proposes a new practical robustness constraint, which improve the robustness at high frequencies, on the bandwidth of a DOB and nominal inertia. Although increasing the bandwidth of a DOB and nominal inertia improves the performance and stability, they are limited by the robustness constraint. Besides, a novel stability analysis method is proposed for reaction force observer (RFOB) based robust force control systems. It is shown that not only the performance, but also the stability changes significantly by the imperfect identification of inertia and torque coefficient. The robustness and stability of a DOB based motion control system are improved by proposing new design tools. The validity of the proposals are verified by experimental results.