Hybrid mechanisms have attracted great attention as a robotic mechanism which merges the advantages of serial and parallel mechanisms. In order to obtain structures of hybrid robot, a very simple yet effective method for structural synthesis of hybrid mechanisms was proposed based on GF set. The basic concept and mathematical operation of GF set, and requirements for rotation based on GF set were firstly introduced. Secondly, this paper presents a method for synthesizing hybrid mechanisms by analyzing characteristics of structure composition of hybrid mechanisms. It is shown that structural synthesis of hybrid mechanisms can be developed by the combination of elements of GF set and rotation axis transfer theorem, number synthesis formulas were set up, and a detailed algorithm of type synthesis for hybrid mechanisms was proposed as well. According to the proposed approach, structural synthesis of 3T2R 5DOF hybrid mechanisms was given in detailed. A lot of novel topological structures of hybrid mechanisms were designed. Meanwhile, a method was proposed to analyze kinematic characteristic of hybrid mechanisms synthesized above. Finally, a 3PRP-a-R&RR hybrid mechanism was synthesized to demonstrate the applicability of the novel method of structural synthesis for hybrid mechanisms. The 3PRP-a-R&RR manipulator performs very well with regard to high stiffness and better accuracy and motion transmission capabilities. At the same time a specific application was given. The research provides a general method for hybrid robot mechanism design and the obtained mechanisms do a beneficial supplement to the hybrid mechanism.