The main purpose of this study was to define book report’s functions and subordinary categories, and to use this to teach writing book reports. We defined writing a book report as a ubiquitous linguistic reaction on continuous situation of writing, especially on books. Under this definition, we tried to approach to the customs of the genre on book reports. In this reason, we studied the notion, elements and the functions of book reports and categorized into subordinary types. Book reports’s main function is on self expression while informational and impressive function is secondary. We categorized book reports as mainly summary type, opinion after summary type, repetitive opinion summary type, mainly opinion type；based on the degree of how summary and opinions are composed, and the amount of information within the text. The significance of this research is that these function based approach method can contribute to the study of genre.