Abstract: Over the last fifty years, Bernsteinian sociology and systemic functional linguistics have shared the same interest in education. The aim of the study is to compare, from a systemic functional approach, the differences between the knowledge structures of pedagogic discourses in different disciplines. The focus of this study is to explore the function of grammatical metaphor in constructing the knowledge structure of texts from different disciplines. A small-size corpus is built by choosing texts from three disciplines, i.e. history, economics and physics. It is hoped that the results of the analysis will help learners to understand the knowledge structure of different types of academic discourse, thus enhancing their reading and writing literacy.