One of the basic and fundamental studies, which have been conducted for stylistic researches, is to identify their genres (varieties) and categorizations. What have been called as letters, missives, correspondences, and epistles (Monshaat or Tarassolat) are one of the epistolary genres needed for conducting of purposeful and widespread studies, as a part of Iranian people’s culture and literature; however, they have not been yet studied as a writing genre. Letter is a communicative medium thereby a person may transfer a message to another one: For instance, to several purposes such as information, ordering, encouragement, and making assured etc. In letter, the audience (receiver) is usually a certain person; having certain audience gives some characteristics to such letters so those properties may make them distinct from other prose genres (scientific, literary, historical, prosimetrum, and story- writing). Identifying such characteristics and categorizations of letters is a subject that the present study deals with it: The questions purposed at this study include: What kinds of special characteristics may exist in letter genre, which makes it distinct from other kinds of prose. How and with what variables one may categorize letters so that their similarities to be revealed in order to organize them in a category ma d also their differences to place them through several genres? The current research is aimed at dealing with writing style of Persian letters, description and categorization, and in order to prepare the ground for conducting stylistic, sociological, studies and researches based on critic discourse analysis and review of power relation within letters and Persian epistolary field and the like. In this research, external texture and the important elements of situational texture have been considered as follows: Author, audience, type of relationship among them and letter action or performance as basis for description and categorization of letters.