Abstarct: While ELT textbooks are developed to improve language learners’ proficiency, a critical analysis of internationally popular textbooks clearly shows that they are guilty of cultural imperialism since they ideologically loaded content, simultaneously they aim at imposing western cultural values and life style as the standard modus vivendi and marginalize language learners’ native culture as substandard and inferior. This study aims at taking the existing hidden curriculum within American English File books into account, to explore and investigate the improper values which are supposed to be injected into the learners’ first culture. The present study used a mixed-methods approach in which the data were obtained both qualitatively and quantitatively. Through open-ended interviews and along the sampling principles of Grounded Theory, the qualitative phase of the study sampled twenty experienced EFL teachers who were asked the research general question. The recorded interviews were transcribed and reviewed over and over in order to extract data which yielded a set of categories – “cultural side effects of American English File books” as the core category paired with some sub-categories such as “Dominance of the Western Culture”, “Propagation of Consumerism and Luxury” and “Imposing Western Life Style” together with “Expanding Imperialism” and “Expanding Secularism. At the same time 145 Iranian EFL learners were randomly selected in the quantitative phase and were asked to fill out a questionnaire including 44 questions baxsed on the cultural values within learners’ first culture and the target culture. The obtained data were analyzed through the use of cross-tabulation and chi-square test of significance. The findings of both phases have revealed that American English file books are not innocent as they were thought to be, for they have the ability to inculcate learners’ first culture, infatuate them and to weaken their religious reservoirs which must be taken into account by policy makers, material developers, EFL practitioners as well as parents and EFL learners.