This paper explore the category of knowledge to be taught in writing instruction, relating to the structure of educational contents. Tradition- ally, the knowledge of writing have been equivalent to instructional contents. But this paper conclude the knowledge consists of the contents structure. There are two factors in the structure of instructional contents, the one is the knowledge, and the other is the function of writing. The function is regarded as an important factor in this paper. The knowledge category make up of declarative knowledge and procedural knowledge, and the function category consist of learning function, thinking function, reflective function, and communicative function. The conclusion this paper is that contents structure have two factor, the knowledge and the function. This standpoint is positive to enlarge of performance-based writing contents, and to clear nature of knowledge to be taught in writing instruction.