Despite extensive research attention that has been paid to second language (SL) or foreign language (FL) learners’ argumentative writing, most research has focused on the structural features characteristic of such writers. There have not been many systematic attempts to identify the quality of argument features SL or FL writers rely on, and how they contribute to the overall writing qualities. This study was designed to examine the relationship between the Toulmin elements, widely used measures of content qualities in arguments which include claims, data, warrants, rebuttals, qualifiers, and backings, and the overall qualities of advanced Korean high school EFL learners’ argumentative writing. Each of the thirty three participants’ argumentative writing was analyzed, applying the Toulmin model, and the results demonstrate that their overall argument qualities were closely related to the uses of the fundamental Toulmin elements, especially data and predicted best by the degree to which each claim was supported with relevant and sufficient data. These findings shed light on the need for instruction on the use of Toulmin elements in enhancing the overall quality of Korean EFL learners’ argumentative writing.