This study compares L2 micro-level rhetoric of supporting detail in argumentative writing produced by learners from three East Asian countries. In order to answer the research question, three hundred TOEFL writing samples, one hundred from each country, were analyzed. In the overall use of supporting detail, the Korean and Chinese writers used Mentioning Advantage (MA) to the highest degree, while the Japanese writers used more Personal Experience (PE). Differences are also found between the three countries in the use of supporting detail sub-categories. The Japanese writers preferred the use of personal expressions, accounting for approximately a half of their supporting details, combining Personal Preference (PP) and Personal Experience (PE). A significant number of Korean and Chinese writers relied on Citation (CIT) and Anecdote (ANC) compared to the Japanese writers; the Korean writers were found to favor the use of objective facts, in that they used research data to support their main ideas, while the Chinese writers showed a greater dependence on traditional expressions such as proverbs. Although Common Knowledge and Truth (CKT) is rarely used by the East Asian writers in this study, the Korean writers displayed a tendency to present it to a greater extent than writers in the other two countries.