Direct manipulations are composed of pointing operations and dragging operations. In order to find the optimum design parameters (such as C/D ratio, moving direction) for the direct manipulation of a GUI(Graphical User Interface), an ergonomic experiment was devised (2*4*3*8 design) to measure the performance of a mouse (Microsoft) and a trackball (Logitech). The results showed that the mouse was more suitable for the direct manipulation (expecially for the dragging operation) than the trackball, and the suitable C/D ratio was 10 (for the mouse) and 16 (for the trackball). Also the movement direction was a determinant factor in trackball performance.