This study aims of highlighting the historical reality of the Abbasid succession problems, which occurred since the beginning of the Abbasid era, and continued during the reign of Caliph Muhammad al-Mahdi(158-169 A.H\775-785 A.D). Specially that his time was a model of a general state that the country lived during many stages of its later time. One of the causes for these problems is the pattern of the hereditary rule which was despotic and autocratic one. The choosing of the crown prince wasn't based on clear bases which lead to many cases in which the pledge to some monarchs has been broken many times. All these things caused many conflicts between the members of the Abbasid royal family. In many times the competition for the crown succession doesn't line in with the religious or ethical rules. The Abbasid Caliph Muhammad al-Mahdi -like his father Caliph Al-Mansur- never hesitate to use the polices of coercion, violence and imprisonment against opponents. Though he himself died because of mentioned policy. One of the aims of this study is elucidation the negative impact that came as a result of succession problem. That was one of the main reasons for the weakness of the Abbasid state later on.
Key words: Succession; Muhammad al-Mahdi; Musa Al-Hadi; Isa bin Musa; Coercion, Dislocation.