Khurramdīnīyyah is one of the greatest religious, political and social movements in Iran in the 9th century C.E. Although this movement is more important than other sectarian movements of that era and there are relatively considerable sources savailable about it, its true nature is Hot clear, primarily due to the fact that these sources mainly speak about it with hostility and disagreement. Moreover, these sources present a confused picture of Khurramdīnīyyah, which is sometimes mingled with the elements from other sects of that period. Mazdakīyyah- a heterodox sect of Sassanid period- probably exercised a powerful influence on Khurramdlnīyyah. It has also been asserted that the latter was the continuation of the former. In fact, after the suppression of Mazdakism by Sassanind kings, the teachings of Mazdak did not disappeared. On the Contrary, through the efforts of his wife and some of his followers who survived from the massacre, these teaching lived on and secretly scattered. It is more likely that Mazdakian teachings were revitalized under the name of Khurramdīnīyyah more than two century later. A dualist belief in two principles, as well as, reincarnation and transmigration are the main doctrines, which Khurramdīnīyyah promoted. Moreover, an extremist form of communal life is attributed to them, based on sharing properties- even their spouses. The lack of a central church and exact religious leadership and the diffusion of the believers in different parts of Iran led Khurramdīnīyyah into dispersion. Several sub-sects, with different names and doctrines, emerged from it, which caused a clear confusion in accounts of Islamic sources about them.