The crown prince position was one of the most influential institutions in the Abbasid caliphate system. In the second period of the Abbasids (232-334 AH), unlike the first period (132-232 AH) when the heirs to the throne were largely independent, the magnitude of influence of this institution on increasing the power of caliphate was lost. Findings of the present research, which uses a descriptive-analytical method to investigate the role of crown princes in weakening the institution of caliphate in the second period of Abbasids, show that earlier significance of crown princes, who during the first period of Abbasids increased the grace and greatness of the caliphate system, was enormously declined due to the domination of the Turks over the crown princes of the second period. The Turks chose young timorous weak-kneed crown princes with the aim of domineering over them. In addition, some women of caliphs’ harem were rivals of Turks in controlling the crown princes. Especially, crown princes' mothers played a major role in installing their children as the caliphs and with their predominance over them were able to direct the sequence of events and thus, prevented the caliphs to deal with affairs. A typical example of these mothers is Sayyidah Shaghab, Al-Muqtadir’s mother, who competed with Turks in governing and ruling in Al-Muqtadir’s period. Total of the abovementioned factors led to weakening crown prince position in the second Abbasid period and consequently, the Abbasid caliphate rule.