Vocabulary knowledge is one of the most important parts of second language acquisition. Thisstudy tried to investigate the effect of portfolio assessment on Iranian high/low self-regulatedEFL learners’ vocabulary knowledge at pre-intermediate level. To carry out this study, 90 femalelearners aged between 16-22 were chosen from Tabarestan language school located in Noshahr,Iran. They took OPT test (Oxford Proficiency Test) and after giving the self-regulationquestionnaire 60 of them were selected as the main participants of the study. They arranged intwo classes, one class consisted of 33 high self-regulated learners and the other class consisted of27 low self-regulated learners. The participants in each class were randomly assigned toexperimental and control group. The pretest was administered to measure participants’vocabulary knowledge. The experimental group received treatment. Both control andexperimental groups attended 7 sessions with the same instructional material. The control groupreceived the traditional assessment while the experimental group received portfolio assessment,i.e., they were asked to provide a folder of writing tasks as portfolio, with providing feedback byteacher. At the end of the course, the posttest was administered. The findings showed that therewas a significant difference between the performance of the experimental group who had theportfolio assessment and the control group who received traditional assessment. Also the resultsrevealed that there was no significant difference between the high and low self-regulated EFLlearners’ vocabulary knowledge. تفاصيل المقالة