Assessment for learning has been extensively researched in the past two decades. However, its applications as a means of classroom-based assessment, especially for promoting teaching and learning of writing, have been underrepresented in English as a Foreign Language (EFL) portfolio settings. This paper aims to critically review the extent to which assessment as learning (AaL), used as an alternative to high-stakes assessment, can support writing instruction and student learning. Drawing upon the evidence from educational assessment and EFL writing, the paper argues that AaL, when advocated in classroom-based portfolios, can promote teacher competence in teaching writing, student motivation for learning, and text improvement. AaL hereby refers to students' ongoing development of cognitive and metacognitive capacity in self-evaluating their writing ability within a portfolio-based environment. Implications regarding how AaL can be successfully utilized to facilitate the development of students' language awareness, self-assessment skills, and self-reflection in EFL portfolio-based classrooms are discussed.