Whether the Achaemenids were Zoroastrians or not has long been one of the questions Iranologists face, studying Achaemenid history. Some Iranologists believe that the Achaemenids were Zoroastrians but others disagree. Discussing Achaemenids’ religion and beliefs, this question is always brought up. One of the reasons that turned this issue into a prominent debate among researchers is Ahura Mazda’s name, mentioned in their inscriptions as a deity and the fact that Achaemenid kings considered themselves worshipers of Ahura Mazda. Nevertheless, their religion remains a question because of their tolerance in relation to other religions. This article intends to study Achaemenids’ beliefs based on a number of experts’ opinions and remaining Achaemenid inscriptions and tablets, such as Darius’ Behistun inscription.