This study investigated the effectiveness of portfolio-based writing assessment in EFL situations. Participants were 40 pre-intermediate young Iranian English learners. They were randomly divided into experimental and control groups of 20 each. The experimental group wrote on five pre-established topics from their coursebook. Their writings were checked in terms of ideas, organization, voice, word choice, sentence fluency, and conventions of writing by two raters. They were given another opportunity to revise their writings to be corrected again. In contrast, the control group wrote only once and their writings were corrected only by their own teacher. The participants were also required to complete a questionnaire to assess their reflection and self-assessment. Results of the study indicate that portfolio-based writing assessment has a positive effect on language learning and writing ability. It also shows that it helps students' self-assessment and almost all students are satisfied with this method of assessment.