One of the historical events that was of special significance in the life of Imam al-Rizā (A.S.) and is noteworthy is the acceptance of Ma’mūn’s invitation for coming to Iran and ultimately accepting heir apparency instead of caliphate. However, why Imam al-Rizā (A.S.) relinquished caliphate and gave his consent for heir apparency to Ma’mūn has its hidden mysteries and historical points. In this writing, the writer tries to present some points that are regarded each as an answer to the question why Imam al-Rizā (A.S.) did not accept Ma’mūn’s proposal for caliphate.