Ever since ancient Greece, up to the Islamic period and even today, one of the most frequently debated issues regarding the ancient Iranian religion, Mazdaism, has been whether or not we face some form of dualism between the good and evil deity at the core of this religion. In this article, the writer starts with the views of Thomas Hyde, the Anglican bishop of Oxford, in the eighteenth century. Based on the scattered texts and accounts available at the dawn of Orientalism, Hyde reaches the conclusion that the Zoroastrian dualism has a philosophical rather than a religious essence. Later, in 1762, Anquentil Duperon presented the Royal Library of France with some manuscripts which were in fact parts of the oral tradition of Mazdaism and not parts of Avesta as it was believed then. Around 1860, the German philologist, Martin Haug, comes to the conclusion that Zoroaster has been a real historical figure whose followers are to blame for the distortion of his faith and the emergence of dualism and hierarchical polytheism in it. In contrast to Haug's perspective, James Darmesteter, who first translated Avesta to French, was of the opinion that Mazdaism is essentially a dualistic religion which has in turn stemmed from an older religion and a polytheistic Hindu-Iranian mythological tradition. Reviewing these points of views in the light of text of Avesta, the writer goes on to examines the views of some of the scholars in the twentieth century such as W.B Henning, I. Gershevitch, Gnoli , and some others on the subject. Finally, he concludes his discussion with a careful and comparative deliberation on the concept of the Millennia in Zoroastrianism and the relation between this concept and the dualism in this religion based on Zoroastrian and Greek texts.