Based on the single-opened-chain(SOC) theory and method for structure design, structure design of parallel mechanism with symmetric 3-DOF was discussed. Some rules of symmetrical minor-mobility mechanism is disclosed. It is an effective method for presenting a structure synthesis approach for forming a 3-DOF parallel manipulator. As examples，5 kinds of 3T and 6 kinds of 3R parallel mechanisms are designed with this method. These mechanisms are topologically symmetric. These mechanisms are deliberately classified according to their topologically characteristics.