The present study aimed to explore the relation between the effect of Thesaurus learning by EFL learners and their listening comprehension ability. To do it, the researcher considers the importance of thesaurus knowledge and how it could possibly affect Iranian EFL learners' listening comprehension ability. Despite the wide range of areas investigated in listening strategy research, there is a lack of research looking specifically at how listening strategy use develops or changes over time in the absence of explicit strategy training. Therefore, to help language learners to improve their listening comprehension ability, special attention must be gained towards vocabulary expansion. With regard to the relationship between one's knowledge of words in a second or foreign language and one's listening comprehension ability, there are also some studies, though few in number, which show that having good lexical knowledge can have a positive effect on one's listening comprehension. In fact, researching spoken vocabulary has always been more difficult than written vocabulary. That is due to two important features which differentiate spoken language from written language.