This study aimed to investigate how incorporating graphic organizers into elementary English writing lessons affected the students’ writing abilities, metacognitive strategies, and affective aspects. Sixty 6th grade students were taught ten writing lessons using graphic organizers and surveyed, and six students were interviewed. The data were quantitatively and qualitatively analyzed. Major findings are as follows. First, the writing test scores of all levels significantly improved after the lessons using graphic organizers. Of the sub-categories of writing (content, organization, vocabulary, grammar, and mechanics), organization scores significantly increased in low level students, and vocabulary scores significantly increased in intermediate and low level students. For high level students, organization and vocabulary scores did not increase significantly. Second, after the lessons, the perception of metacognitive strategies improved for students of every level. Third, the affective aspects increased for students of every level. The students’ learning motivation increased much more than their confidence and interest, although intermediate students’ motivation did not significantly increase. Overall, using graphic organizers were effective in the students’ cognitive and affective development in writing. In particular, low level students improved the most, indicating the need for considering student levels when using graphic organizers in writing lessons. Further implications regarding the use of graphic organizers in writing lessons are discussed.