Abstract: The method to formulate the closed form solution to the translational workspace boundary of 6 PSS parallel manipulators is propos. With a understanding of the kinematic characteristics of the manipulator, the workspace boundary as a whole is formulated as the intersection of three subsets, i.e. two subsets produced by the intersection of six spherical surfaces, and one subset produced by the intersection of six elliptical cylindrical surfaces. The primary workspace is also defined and the influence of the structural parameters on such workspace is discussed.