Mashoori is a dialectal variation of southeast of Khuzestan Province, Iran. This dialect is spoken in Mahshahr and its regional neighborhood and, inherited from the Old Southwest Iranian Languages. It is somehow rich from the phonological perspective. One interesting phonological process applied in MD is the alteration of consonantal /b/ and /f/ to the semivowel [w] that is assumed to be kind of lenition process. /w/ in the Old Southwest Persian changed historically to [g], [b] and [w] in Persian Dari and to [g], [b] and [v] in Modern Standard Farsi. The data required for this descriptive-analysis study make use of a dialectal report manual gathered by five native speakers of MD, plus data collected from previously done related researches on this process within the theoretical framework of Generative Phonology. Findings render that this process is remarkably different from what is going on in Modern Persian in a way that seems plausible to choose /w/ as the underlying representation, changing to /b/, /f/ and /v/ in Modern Standard Persian under the fortition process.