The portfolio assessment is a purposeful and systematic collection of students’ work that is intended to show progress over time. The researchers employed the Oxford quick placement test, autonomy questionnaire, and topic-based paragraph writings as the pretest and posttest. The design of the study was quasiexperimental. To this end, researchers chose 120 learners with the convenience sampling method, which were 60 learners at the upper-intermediate and 60 learners at the advanced level. Participants were divided into two homogeneous groups (30 learners) such that there were two groups at the upper-intermediate level and two groups at an advanced level as experimental and control groups. A piloted writing pretest was given to both groups before the treatment. The experimental group had received the treatment of four writing tasks to perform. In order to address the research questions, descriptive statistics and covariant analysis were used. Based on the results achieved from the first research question, portfolio assessment has a positive significant effect on Iranian EFL learners’ autonomy on both upper-intermediate and advanced levels. The results also indicated that portfolio assessment has a positive significant effect on Iranian EFL learners’ writing skills on both upper-intermediate and advanced levels.