In the administrative body of the Islamic Caliphate (Umayyads and Abbasids), the Post Bureau had an important function in obtaining news as well as information exchange and maintaining the security of the caliphate territory. The need for governing the vast caliphate territory, the acquisition of news and events from the states and their transfer to the Darul-Khilafah, the supervision of the brokers’ functions, doing the postal services and caring for the spy activities, made this Bureau important in the administrative organization of the Islamic Caliphate. In this research, which is of a historical type, in the collection stage by the library method and in the explanatory stage by an analytical descriptive method, the status of the Post Bureau in the Islamic caliphate and its functions in the post and communication services as well as intelligence and security affairs are investigated. The findings of the research show that the political- security function of the Post Bureau elevated its position and importance in the caliphate system to the extent that the owners of the Post Bureau enjoyed high status in the caliphs' court and the internal security and the maintenance of the caliphate power were linked to the functioning of this institution.