In this paper, the orientation-singularity and orientation capability analyses of the Stewart-Gough manipulator are presented. Based on a particular kind of orientation representation of the manipulator, a polynomial expression of degree thirteen is derived, which represents the orientation-singularity locus of the manipulator at a given position, and examples are given to illustrate the result. The description of the orientation capability of the manipulator and the orientation capability index are discussed. In addition, effects of the geometry parameters of the moving platform, including the central angle, the ratio of the moving platform to the base one and the position of the moving platform on the orientation capability of the manipulator are also presented in this paper.Copyright © 2005 by ASME