This article centers around a decisive factor in murdering Esfandiar which has not been noticed yet, and tries to find out how Fate supported by Mithra determines the destiny of Esfandiar. To keep promises, is one of the duties of Mithra who punishes those who break promises. Goshtasb is among the ones to be punished because he has repeatedly promised the kingdom to Esfandiar but has not kept his promise. As the final consequence, Rostam, according to Mehryasht, kills Esfandiar in a battle.