This study examined developmental differences in argumentative writing performance. Two-hundred students (Grades 6, 8, 10) wrote about \'going to school every Saturday.\' Two raters assessed each written product using 5-point rating scales with 11 items, which were categorized into \'content development,\' \'content organization,\' and \'expression.\' Data were analyzed by multivariate analyses of variance with gender and grade as independent variable and mean scores as dependent variable. The results are summarized as follows. First, in the clarity of opinion, female students in Gr. 6 and 8 excelled those in Gr. 10; however, such differences did not appear in male students. In the remaining content development items, whereas male students\' writing performance improved with grade, female students did not show such developmental tendency. Second, in all of the organization items, male students\' writing performance improved with grade, but such developmental differences were not significant in female students. Third, in all of the expression items, regardless of gender, students\' writing performance improved with grade. Implications of the results for writing instruction and for further research on examining the quality of argumentative text written at each stage of the writing process and that written on various topics were discussed.