During several centuries, what has remained of the fasinating ritual of “trial by ordeal” is swearing. Trial by ordeal was a common ritual among the nations and religions, and was considered a judicium Dei and the ultimate test to distinguish innocencent from guilty. Some instances of it have been cited in Vis and Ramin, Masnavi, Solaman va Absal, Makhzan al-Asrar, History of Yaqoubi, Haft Eqlim, Asar al Belad va Akhbar al Ebad, Tadhkirat-ul-Awliya, Avesta, Torah, Pahlavi texts and code of Hammurabi inscription. “Passing of Siyavash through the fire” is the most familiar example of trial by ordeal in the collective memory of Iranians. The reasons why the “myth of trial by ordeal” in Shahnameh is an outstanding example, is the main subject of the present article. So, at first, the significance of fire among the ancient nations and religions is considered. Then, after citing some tests of “passing through the fire” in historical, religious and literary texts, the myth of “passing of Siyavash through the fire” as the most impressive and lasting instance is introduced.