Metaphors have different expressions in cognitive and systematic-functional linguistics, which are conceptual metaphors and ideational metaphors. Conceptual metaphors belong to traditional vocabulary metaphors, and ideational grammatical metaphors are pointed out by Halliday--the main representative of systemic-functional theory. This paper elaborates on the specific grammatical feature of EST （English for Science and Technology） text--＂nominalization of processes＂ to reveal the source of ideational metaphors. From the analysis of some examples, it is found that nominalization as a grammatical metaphor expresses the transference between process types, so it reveals the essence of ideational grammatical metaphors. Meanwhile, the comparison made between grammatical metaphors and vocabulary metaphors reflects the similarities between conceptual metaphors and ideational metaphors. In this way, this paper comes to the conclusion that the combination of the study methods of metaphors from both cognitive and systemic-functional perspectives is the best way to uncover the basic characteristic of human thinking.
