Task representation denotes students’ interpretation in which what a learning or assessment task required them to do. An argumentative integrated writing task which involves the use of reading materials as claims or evidences for composing an essay, makes the role of task representation more critical than others, as writers may be confused with whether their task is to focus on synthesizing the reading materials that they comprehend, or expressing their own views. With the aim of exploring the characteristics of task representation and its association with integrated writing, this study invited 474 secondary four students from Hong Kong to participate in think aloud writing protocol followed by stimulated recall interview (36 participants), and complete an integrated writing task and a questionnaire (438 participants). Three factors of the task representation were identified as source use, rhetorical purpose and text format, and significant positive correlations were found between the three factors and integrated writing performance. Theoretical and pedagogical implications are discussed.