In order to have a correct strategy of function partitioning in product design, the function partitioning and the function granulating were discussed based on the studying on reasoning process of function optimal design. The rules of function partitioning were introduced. Further, a decoupling method in function partitioning was put forward, and a rule of granulating controlling was put forward. Then correctness inspecting of function partitioning was discussed by establishing a function structure of an internal-combustion engine. The results show that the function partitioning should comply with the abstract rule, independence rule and granulating controlling rule; Decoupling is possible by a correct strategy of function partitioning; The function need not be partitioned to the minimal units (the function units or basic functions), and when the partitioning is ended depends on the experiences and the knowledge of designers to a large extent.