Metathesis as a phonological phenomenon in order to reverse segments has been focus of many studies. Metathesis according to the range of application can be sporadic or regular. It links synchronic studies of language to diachronic ones and thereby has some consequences for phonological theory. The central goal of this paper is to elucidate the conditions under which metathesis occurs synchronically in modern Persian and diachronically in Pahlavi (Middle Iranian Persian). In this paper, first metathesis is viewed from different aspects especially a historical one. And particular attention is paid to its causes and contexts. Then, metathesis is studied within Optimality Theory, using constraints such as Syllable Contact Law (SCL), Sonority Sequence Principle (SSP),… Also it was found that segments don't behave identically in the metathesis process. Phoneme /r/ is the most frequent and /z/ is the least one found in Persian.