Abstract The main purpose of this research project was to investigate the process of interlanguage phonology development in Persian EFL learners. Optimality Theory (OT) was chosen as the theoretical framework of this study. In this study the researcher adopted the Full Access-Full Transfer hypothesis and assumed that Persian speakers begin their second language (L2) phonology learning with a full transfer of the constraint ranking from their first language (L1). It was also assumed that the constraints are part of Universal Grammar and that the learners have full access to these constraints. First, it was shown that constraints ranking of English and Persian languages regarding their syllable structures are different. Then, based on the findings of previous Optimality Theoretic studies, and considering the areas of similarity and differences between constraint rankings of the two languages, some predictions were made about the type and rate of errors made by the participants. Finally, these predictions were tested and it was found out that the errors committed by the subjects on both onset and coda positions were, to a large extent, consistent with the predictions. On the whole, it can be concluded that OT could successfully account for L2 language perception and productions by the participants and provides explicit explanation about the stages of interlanguage phonology development.
