The dimensional synthesis of the mechanism of an in-parallel actuated manipulator should take into account high motion transmissibility and avoidance of the area surrounding singular points as well as the singular points in the prescribed working space. In this paper we propose a new method based on the Monte-Carlo technique to obtain the kinematic constants which assure high motion transmissibility. This method has been applied to dimensional synthesis of a 6-RSS type spatial in-parallel actuated manipulator with six degrees of freedom. The effectiveness of the proposed method has also been investigated by comparison of the results to those obtained by the conventional method. These results have been used to develop a manipulator driven by direct drive motors. The position repeatability has been experimentally investigated and the result reveals that the manipulator can be used to realize precise positioning superior to those of conventional industrial robots.