Although the Imamiyya holds that the Imams have been appointed by God to rule the Community and that they are the complementary elements of Religion as a divinely guided persons, if the lives of the Imams are examined, some points contrary to this theory can be easily seen. As an example of this, in this article we dealt with the matter of the heirship of eighth Imam Ali al-Ridâ to the Abbasid Caliph Ma’mûn, which seems contrary to the mission undertaken by the Imams. So, we aimed to identify to what extent the points presupposed in the Imamat theory have been corresponded with the practices of Ali al-Rida. Accordingly in the article, the period ofAli al-Rida, his heirship to the Caliph Ma’nun and the reactions to it, the explanations of Imamites about this event and its practical value are respectively discussed