The present study investigated the effect of bilingualism on Iranian EFL learners' vocabulary retention to see the probable differences between Iranian monolingual and bilingual learners of English regarding vocabulary retention. The main concern of the present study was to examine whether bilingual and monolingual learners of English differ in keeping and retaining words. To carry out this study, a total of 120 female participants aged between15 and 18 at pre-intermediate level of English at

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... fir language school in Saveh were randomly selected from among two groups of Turkish-Persian bilinguals and Persian monolinguals. A general English proficiency test, a questionnaire and a researcher-made vocabulary test in three phases as pre-test, posttest, and delayed post-test were administered to both mono and bilingual subjects. Soon after the pre-test, the students got treatment through color pictures. In fact picture recognition was used as an instrument for vocabulary retention (Makato Yoshi, Jeffra &Flaitz, 2002). Twenty days after treatment the second vocabulary test (post-test) was administered and the effect of treatment was observed. Finally, the delayed posttest (after a 7-day gap) was administered to see how monolingual and bilingual learners remembered and retained the words. Statistical analysis including descriptive statistics and inferential statistics (repeated measures-ANOVA) revealed that firstly, monolinguals and bilinguals differed significantly in their performance on language proficiency test. In fact bilinguals outperformed monolinguals in doing language proficiency test. Secondly, there was difference between monolingual and bilinguals learners in vocabulary retention: bilinguals had an enhanced ability to retain the words as compared to monolinguals.