This study was intended to describe the relation between students' reasoning and their ethnicities. Comparison between the argumentative writings written by Javanese and Bataknese students - two of the ethnics found in Indonesia- was conducted to seek shared and variation of reasoning between the two ethnics. Interpretation on the results of such comparison was made in order to find out the relation between reasoning and ethnicity. To achieve the objective, a descriptive qualitative research design was applied. Twenty students consisting of 10 Javanese and 10 Bataknese were involved in the study. They were assigned to write 20 argumentative writing. Qualitative data analysis was applied to analyze the 20 argumentative writing. The result of the study showed that both Javanese and Bataknese students shared common patterns of arguments in terms of text structure, reasoning, and affective appeal. Meanwhile, difference in cultural value between Bataknese and Javanese students caused minor variation (differences) in text structure of argumentative writing in two aspects namely in developing thesis statement and drawing conclusion. Ethnicity's relation to variation in argumentative writing is limited to the two aspects: to the writing of thesis statement and drawing conclusion.