abstract this study aimed at investigating the effect of bilingual teaching of cognate words (persian-english) on iranian upper intermediate efl learners’ knowledge of lexical development. for this purpose,100 subjects participated in this study out of which 40 learners were selected for this study and they were assigned into two groups, control and experimental. cross-language cognates (words with similar form and meaning in different languages) are of special interest for designing a model in tefl, since they help teacher make the teaching of english vocabularies a joyful and lasting effect for persian students. true cognates are the result of kinship relations across languages, or borrowings. false and true cognates might be found in nearly all languages with any kind of relation to other languages. there are still some "real" cognates found in the lexicon of persian and english. then the data was analyzed statistically through ancova. the results of the study showed that the learners’ lexical knowledge was improved when they are presented with bilingual teaching of cognate words (persian-english). the conclusions of this study will provide new insights into the linguistic and the communication problems derived from a misuse of these lexical items. the study of false friends and true cognates has several implications for contrastive analysts, error analysts, translators, foreign language teachers and learners, curriculum designers, as well as lexicographers and lexicologists. key words: cognate word, efl students, lexical development