The present study sought to investigate the effect of portfolio assessment on idiom competence of Iranian EFL learners. For the purpose of this study, 30 students from upper-intermediate level of English proficiency took part in this study. They were chosen through convenience sampling from a language institute in Rasht, Iran. They were randomly divided into experimental and control groups. A TOEFL test and a test of idioms were given to the students to ensure their homogeneity in terms of language proficiency and knowledge of idioms, respectively. The experimental group was intended to create a portfolio and put their writing samples, in which idioms were used, in the portfolio. They were involved in the process of self-and-peer assessment. The teacher also provided them with feedback and comments. However, the control group received a kind of traditional instruction. In other words, the control group used the idioms in their writing without receiving any comments and delivered it to their teacher to be scored. The treatment lasted for 10 sessions and a post-test was administered in the end. Independent samples t-tests were used to analyze the data gathered from the pretests and the posttest. The findings indicated that there was a statistically significant difference between the two groups in terms of idioms and portfolio was found to be able to improve students' knowledge of idioms. The results of this study have some implications for teaching and learning idioms.