Capitalizing on Hulstijn and Laufer’s (2001) notion of Involvement Load Hypothesis, this study aimed to investigate the impact of task-induced involvement load on Iranian EFL learners’ retention of idiomatic expressions. The main concern of present study is to explore if principles of involvement load hypothesis hold true when applied to learning idioms. The research questions formulated to achieve the objectives include: 1.What is EFL learners’ retention rate of idiomatic expressions for the reading-comprehension task with involvement index 1? 2. What is EFL learners’ retention rate of idiomatic expressions for the fill-in-the blanks task with involvement index 2? 3. What is EFL learners’ retention rate of idiomatic expressions for the composition task with involvement index 3? 4. Does Involvement Load play any significant role in the retention of idioms of Iranian EFL learners? Given the fact that the depth of processing a word is the key component in vocabulary learning, it could be assumed that composition tasks designed to develop idiomatic expressions in EFL learners, enjoy much greater retention rate, hence attesting to the effectiveness of involvement load on the retention of idiomatic expressions. To this end, 60 female upper-intermediate Iranian EFL students in an English language institute in Tehran took part in the study. Twenty idioms were taught to three groups of students through tasks that induced different levels of involvement load, namely reading comprehension, fill in the blanks and composition tasks. To conduct the statistical analysis, fifteen idioms were picked from among the ones presented in the pretest, so that learners’ prior knowledge of some idioms was taken care of. The retention of the chosen idiomatic expressions was then tested via an immediate and a delayed post-test. MANOVA was run to compare the groups’ performance on the three tasks on the posttest and delayed posttest. Scheffe’s test was then used as a post-hoc comparison test to compare the groups on posttest and delayed posttest. The results pointed to a direct relationship between the involvement as induced by the tasks and participants’ retention of idioms, implying the fact that the higher involvement load of a task, the higher the retention of the task will be. Accordingly, the findings indicated that retention rate was the highest in the composition task, lower in the fill in the blanks, and the lowest in the reading comprehension task. The study has implications for task analysis, task instruction, and task design; Curriculum developers may benefit from the findings of this study when engaged in designing courses related to idiomatic expressions. English teachers could be more cognizant of varying involvement indexes in tasks and their effect on the retention rate of their learners. Eventually, language learners will be informed of the optimal amount of time and effort needed for mastering idiomatic expressions.