Introduction: In the study of ancient Persian religions, three religions are prominent; they are Zorvani religion, Mithraism or Kish-e Mehr and Zoroastrianism. Mithraism has close ties with Zorvani religion. It also had two major rituals, one was the sacrifice of bulls and the other was drinking juice and extract of Hoom, a sacred plant of ancient times. Hoom is considered to be ephedra vulgaris, a small plant with yellow flowers.There are many sacred trees and plants in religious and mythological narratives of many religions which usually represent the living universe continually rejuvenating itself. There are also mysterious relationships between human beings and trees and plants; some tree or plant is considered as the mythological ancestor of a tribe.The word Hoom (Avestan haoma, Middle Persian hom) is mentioned in different parts of Avesta. Hoom was a golden plant and a drink was extracted from it which enjoyed a special place in Mithraic ceremonies. With the advent of Zoroastrianism Hoom was regarded negatively but later on regained its importance as depicted in adding a part to Avesta named Hoom Yasht in which Haoma is manifested as: 1) a golden plant, 2) an intoxicating drink, 3) a man, 4) a god.In this paper the role of this mythical plant is discussed in the formation of some of the narratives of Persian Epic, Shahnameh. This study uses textual analysis of ancient religious texts and Shahnameh to discuss the mythological and archetypal elements of a classical Persian epic. Discussion: Hoom is only once mentioned directly in Ferdowsi's Shahnameh, that is the appearance of a woolen-clad pious man named Hoom. The story relates how during his prayers at the temple, Hoom hears a man groaning in a cave. Finding out that he is Afrasiyab, the enemy of Iran, he ties his arms with his girdle. However, Afrasiyab tricks Hoom and runs away, but with the help of Kai Khosro he is captured again, then Hoom disappears.In Shahnameh, Simorgh (phoenix) the legendary bird advises Rostam to use a special plant to be mixed with milk and musk in order to cure Rudabeh's wound. In Rig Veda and Avesta the mixing of Hoom and milk and pounding them in a mortar are mentioned many times. It is also mentioned that this plant is medicinal. Hoom is said to grow on Alborz Mountain where Simorgh used to live. Hoom can also be traced in the story of Rostam and Sohrab. Rostam not recognizing his son, Sohrab, wounds him; he asks Kai Kavus for an antidote with wine to cure Sohrab. The description of antidote is very much like that of Hoom. Another instance is the story of the death of the invincible Esfandiar in war with Rostam. Simorgh advises Rostam to choose an arrow and to dip it in raz water (grape juice) or wine which is supposed to be fatal. Again the description and the toxicity of the drink is very much that of Hoom.Conclusion: The ritual-mythical relationship of Simorgh and Hoom in Shahnameh is mostly related to Rostam's family. Simorgh and Hoom are also related to Mithraism and the worship of Mehr, an older religion that Rostam is its chief priest. That is why Esfandiar as the upholder of the new Zoroastrian religion kills Simorgh that with the help Hoom is the protector of Mehr and the followers of Mehr.