Nowadays, writing has received a great degree of attention not only because it plays a significant role in transforming knowledge and learning but also in fostering creativity and when acquiring of a special language skill is seen as important, its assessment becomes important as well and writing is no exception. This study intended to investigate the effect of portfolio assessment technique as a teaching, learning and assessment tool on writing performance of EFL learners. Writing sub-skills has also been taken into account. To this end, forty Iranian EFL learners who were all English teaching majors were randomly divided into two groups: experimental (n = 20) and control (n = 20). The experimental group received the treatment i.e. portfolio assessment while the control group underwent the traditional approach of writing assessment. The result of statistical analysis indicated that the students in experimental group outperformed the students in control group in their writing performance and its sub-skills of focus, elaboration, organization, conventions and vocabulary. The findings suggest that portfolio assessment technique improves writing ability of the students. The results have also some implications for assessment, teaching and learning of L2 writing.