Task-based language assessment (TBLA) is a framework for language testing that takes the task as the fundamental unit for assessment and testing. This study was designed to investigate the impact of TBLA on Iranian Male and Female EFL learners’ idiomatic knowledge. To fulfill the objective, 40 learners ranging from 18 to 25 in age and studying at Bayan English Institute in Rasht, Iran, were selected from among 100 participants based on their performance on quick placement test (QPT). The study employed a true-experimental design in which the participants were randomly assigned to two experimental groups and two control groups. First, a pretest of idioms was administered. Then, the experimental groups received a sixteen-session treatment that was the treatment of Task-Based Assessment. Meanwhile, the control groups received a sixteen-session placebo. A posttest of idioms was then administered to all the participants. The use of UNIANOVA revealed that the experimental groups outperformed the control groups in learning idioms. Based on the findings of the present study, all stakeholders, particularly material developers and teachers are proposed to include idioms in language teaching so that remarkable opportunities for improving idiomatic knowledge among learners would be provided.
