Recent requirements for fast and accurate motion in industrial robot manipulators demand more advanced control techniques. Past techniques, such as the resolved acceleration method and nonlinear feedback control, etc. require detailed information on mechanical parameters. In these methods, the modelling errors adversely effect manipulator motion. To overcome this problem, this paper proposes a method for constructing nominal dynamic modelling for a robotic arm in the operational space. When the proposed method realizes a disturbance observer in the operational space, individual coordinate motions can be decoupled and independently controlled. Further, individual motion in the operational space coordinate is driven as if only nominally fixed mechanical dynamics in the operational space existed. As a result, the method enables to construct the control law without any calibration for the mechanical parameters. Finally, we present experimental results from an implementation of the proposed method.