The purpose of this article is to investigate lenition in Sistani dialect in generative phonologyframe. At first the short description of Sistani dialect, Methodology and generative phonology is represented, and then the phonological process lenition in Sistani in comparison with Persian is deal with. For this purpose, all alternation in respect of lenition, in Sistani in comparison with Persian, were extracted then according to some criteria , it is determined whichform is underlying representation; for instance, in Sistani, bilabial consonant such as [b] in the coda position of syllable is converted to [f],[v],[w].likeɑb→ ow. According to” Frequency of occurrence, it is proved ̸ɑb̸ is underlying representation.

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