Since antiquity, man has been in search of his life’s fountainhead and the origin of creation and with reflection on this world has tried to unravel its mystery. After coming of Zoroaster in Iran and appearance of Mazdai rite, Ormazd and Ahriman have been regarded as two poles of creation and creators of good and evil. However, there are evidences and documents demonstrating that before believing in dualism, the idea of monotheism based on the unity of time preoccupied the mind of Iranians. Belief in Zarvan, god of time, had numerous consequences more than anything else, belief in fate (destiny), and predestined lot seized the mind of Persians for centuries. This idea penetrated through the minds of Iranian poets, thinkers and philosophers. Among them, Ferdowsi, the great Iranian epic poet, like others was no exception. It is assumed that determinism in Shahnameh more than anything else has roots in Zarvanism.In this article, first the origins of the idea and philosophy of Zarvanism in Old Persia, before and after Zoroaster is sought, then with citing evidences from Shahnameh, the roots of Zarvani Philosophy is pointed out.