This dissertation Comprises four chapters. Chapter 1 discusses the theoretical framework and methodology on which this research is based.A brief review of the related literature is also included. Chapter 2 provides critical review of 47 different prose works written in 12th and 13th centuries.In this chapter linguistic pecularities of each text are examined individually.Chapter 3 sketches the phonological system of Persian in the 12th and 13th centuries.Chapter 4 deals with the morphology and syntax common to all texts already discussed.