In such cases as conveying bodies by use of motions of coupler links, a kinematic synthesis has to be made considering the coupler directions. In this paper, dimensional analysis and synthesis of a six-link Watt mechanism are carried out. The parameters of the mechanism are split into the ones which determine the shape of the coupler curve or the direction of the coupler and the ones which determine the position and magnitude of the curve. Moreover they are qualitatively classified into three kinds. By use of these parameters, an optimum synthesis of a Watt mechanism is carried out such as to satisfy every planar path with two directions of a body, without any trial and error.