While research on portfolio assessment (PA) has focused largely on the summative aspects of writing assessment, not much has been done to find out its formative potential. Drawing upon student questionnaires and student and teacher interview data, this paper aims to explore the formative functions of PA and, specifically, how the formative potential of PA can be better utilized in the EFL writing classroom. The findings of the study indicate that although students responded positively to the formative aspects of PA, they still preferred summative grading and tended to believe that grades were the bestway to inform their current standards of writing. The paper ends with a discussion on how the formative aspects of PA could be strengthened and how PA can integrate teaching and assessment to benefit students’ learning of writing. Copyright © 2009 The Author. Published by Oxford University Press.