Mazdak appearance on Sassanid era is one of the most important historical events of Persian history which has certain complexity and ambiguity. This event has narrated extensively by Ferdowsi in Shahnameh and the historians and researchers of Shahnameh have always referenced to Shahnameh in their analysis of this story. They also have used the contents of the national epic in their reviews. But the wonder is that neither Iranian researchers nor the orientalists referred to several fundamental differences between the narrative of Shahnameh and other histories about this event. The most important difference is the change of a part of Ghobad history after the emergence of Mazdak in Shahnameh. In this paper, the advent of Mazdak is studied critically in Shahnameh and other historical sources.