Abstract: The concept of grammatical metaphor (GM) stems from stratification of language, but there has always lacked a cognitive interpretation of its generating mechanism. This paper tries to explore the generating process of GM by adopting concepts of mapping and blending from cognitive linguistics. It argues that the generation per se is the result of mapping and blending between representation modes of “inner” and “outer” realms of experience in clauses, i.e. the transference between relational and operational modes. It is found that GMs have two general types that may be particularized into 24 categories of mapping and blending. The discovery may shed light on further understanding of the generating mechanism of GM and help clear some possible confusion incurred by different categorizations of GM, and in doing so it finds a way to integrate the cognitive conceptual metaphor into the GM.