this study was conducted to investigate the impact of portfolio assessment as a process-oriented assessment mechanism on iranian efl students’ english writing and its subskills of focus, elaboration, organization, conventions, and vocabulary. out of ninety juniors majoring in english literature and translation at the university of isfahan, sixty one of them who were at the same level of writing ability were chosen as the participants in the main phase of the study. they were divided into two groups. the experimental group received the treatment i.e. portfolio assessment, while the control group underwent the traditional assessment. the results of data analysis revealed that the students in the portfolio assessment group outperformed the students in the control group in their overall writing ability and in the subskills of focus, elaboration, organization, and vocabulary. to triangulate the quantitative data, students views and reflections about portfolio assessment were elicited via portfolio contribution questionnaire and individual semi-structured interviews. the results of analysis of qualitative data showed that students perceived improvement in overall writing and its subskills; however, there were contradictory views about which subskill they improved most which might suggest the room the participants found in the experimental group to individualize learning. additionally, the qualitative data underwent thematic analysis and the major extracted themes i.e. writing achievement, use of reflection, and awareness of writing were discussed in detail. in sum, the findings suggest that portfolio assessment empowers students learning of english writing, hence emphasizing the formative potential of portfolio assessment in efl classes. the results have some implications for curriculum development and teaching and assessment of l2 writing.