Due to the scarcity of quantitative studies as to the impact of portfolio assessment on EFL students’ writing ability and the significant impact of the interaction between portfolio assessment and self-regulation strategy, the present study aimed to explore whether portfolio assessment has any significant effect on improving Bachelor of Arts (BA) English as a foreign language (EFL) students’ paragraph writing ability, and whether this effect differs within high/low self-regulated learners or not. To do so, 60 intermediate female students were chosen out of 145 learners through the administration of a standard version of Oxford Placement Test (OPT). The participants were randomly assigned into one control (30 participants) and one experimental group (30 participants). The experimental group was assigned into two groups of high and low self-regulated learners, (15 participants for each group), based on Magno’s (2009) Academic Self-regulated Learning Scale (A-SRL-S) questionnaire. Participants of the control group were taught and assessed based on traditional teaching and assessment, whereas those in the experimental group were taught and assessed via portfolio-based instruction and assessment techniques. The analysis of the results of the study revealed that portfolio assessment has a significant effect on improving writing ability (p=0.001). The results also showed that high self-regulated learners have taken more advantage of portfolio assessment than the low self-regulated ones (p = 0.000). The results obtained from the present study can have beneficial contributions to teaching, curriculum development, and testing.
