This paper systematically studies structure synthesis and dimension optimization of XYZ flexure parallel mechanisms (FPMs) with large-motion and decoupled kinematic structure. Different from structure synthesis of rigid-body mechanisms, structure synthesis of flexure mechanisms is constrained by the limitations inherent in flexure mechanisms. These limitations are investigated and summarized as the structure constraints. With consideration of these structure constraints, the configurations of the decoupled XYZ-FPMs are synthesized using the Screw Theory. The synthesized XYZ-FPMs also possess large motion range, due to integration of a new type of large-motion prismatic joint designed in this paper. The stiffness models of the synthesized XYZ-FPMs are formulated. A 3-PPP XYZFPM is developed as the case of the studies of structure synthesis and stiffness modeling.