THIS STUDY AIMED AT INVESTIGATING HOW 30 PERSIAN LEARNERS OF ENGLISH AT INTERMEDIATE LEVEL USED THEIR FIRST LANGUAGE KNOWLEDGE TO COMPREHEND AND PRODUCE THREE TYPES OF ENGLISH IDIOMS, WHICH WERE INCLUDED AS COMPOSITIONAL, SEMI-COMPOSITIONAL, AND NON-COMPOSITIONAL IDIOMS. THE LEARNERS WERE DIVIDED INTO TWO GROUPS; ONE EXPERIMENTAL AND ONE CONTROL. A PRE-TEST WHICH INCLUDED BOTH COMPREHENSION AND PRODUCTION TESTS WERE ADMINISTERED TO EACH GROUP. THEN, EXPERIMENTAL GROUP WAS RECEIVED TREATMENT IN THE FORM OF SMALL CONVERSATIONS, BUT CONTROL GROUP WAS NOT RECEIVED ANY TREATMENT. IN THE LAST SESSION BOTH GROUPS WERE RECEIVED IMMEDIATE POST-TEST AND AFTER ONE MONTH, A DELAYED POST-TEST WAS TAKEN. FOR THE ANALYSIS OF DATA A T-TEST AND ANOVA WERE EMPLOYED. RESULTS IN BOTH GROUPS SHOWED THAT COMPOSITIONAL IDIOMS HAD THE HIGHEST CORRECT RESPONSES, FOLLOWED BY SEMI-COMPOSITIONAL IDIOMS, AND NON-COMPOSITIONAL IDIOMS WERE THE MOST DIFFICULT TO COMPREHEND AND PRODUCE. IN ADDITION, IN BOTH GROUPS LEARNERS PERFORMED BETTER IN COMPREHENSION RATHER THAN PRODUCTION TEST.