Although the use of Grammatical Metaphor (GM) has been regarded as one of the representative features of academic writing in the framework of Systemic Functional Linguistics (SFL), not many studies have explored its role in second or foreign language writers’ writing quality. This study aimed to discover the potential relationship between GM development and argumentative writing quality of advanced Korean EFL learners, while controlling for their overall language proficiency and reading comprehension ability. The findings from this study unveiled significant contribution of the use of both premature and mature GM and also identified transcategorization as the specific GM that has relatively stronger predictability of writing quality. The results from current research yield meaningful pedagogical implications for writing instruction in Korean EFL contexts.