It is generally accepted that Ferdowsi relied on a late Sasanian chronological history, known as Khwad?yn?mag, in order to construct the narrative of that dynasty’s history, which takes up the latter portion of his Shahnameh. Since no manuscript of this text has been found, it is difficult to judge the extent to which the author incorporated its contents and what revisions he made. One episode in his work can be traced to a specific, known pre-Islamic text. K?rn?mag-e Ardash?r-e P?bag?n contains a narrative which is almost identical to the life of Ardashir I in the Shahnameh. The similarities between these two works provide an opportunity to draw conclusions regarding Ferdowsi’s approach to written sources and his understanding of his own voice as an author. This paper briefly demonstrates the relationship between these works, focusing on the rhetorical strategy and ideology implicit in each narrative. The rhetoric of legitimation of Ardashir’s rule in various texts of the Sasanian era, such as the “Letter of Tansar” and D?nkard, will be examined and compared with Ferdowsi's narrative of Ardashir. Arguments borrowed into this work from the earlier text include bloodline connections to past dynasties, affirmation of the Zoroastrian faith, and royal marriages. In this way, the paper will respond to important questions on the nature of Ferdowsi’s sources, such as those made by Dick Davis, Mahmoud Omidsalar, and Olga Davidson. The paper examines the rhetorical motivations of the anonymous Sasanian-era author of the K?rn?mag by considering theories regarding the dynasty’s political and social structures in the works of Arthur Christensen and Parvaneh Pourshariati. Ultimately, the paper demonstrates the unique motivations of each author; while the earlier work sought to reconcile powerful factions in pre-Islamic society to the ruling order and defend its claims to power, Ferdowsi intended to portray a thread of perpetual continuity in Iranian kingship from the beginning of time. To achieve his goal, Ferdowsi borrows heavily from the the K?rn?mag because its arguments support his purpose.