Fostat, the first Islamic capital of Egypt, was founded in the years 17-20AH along with Babelyon by raising tents of militaries as the conqueror of Egypt. This ancient Islamic city that its military nature dominated over its urban structures for many years was the first Islamic city and the capital of Egypt, and the governor’ s seat (dar-ol-hokumah and dar-ol-emarah) of Muslim Emirs. In their haste to choose a construction site for the city and their unfamiliarity with urbanization techniques, Muslim conquerors neglected some important issues, which caused that Fostat could not be erected as a sustainable city in the history of Egypt. Thus, after construction of “ the Qaherat Almoezzi” , its glory was rapidly declined and continued its life as one of the old neighborhoods of Egypt’ s capital, along with this new capital of Fatimids (i. e. Cairo). There are various reasons due to which Fostat was unable to maintain its urban situation comparing Cairo, however, the environmental factors as well as in consistency of its physical space have been neglected by the recent researches in this regard. In this study, which uses a descriptive method, collects data from historical references and benefits from analytical/deductive method, we aim to answer this main question: what are the environmental differences between Fostat and the Qaherat Almoezzi as two neighboring capitals? The outcome of this study showed that the environmental differences, and bad climate of Fostat are due to improper geographical and physical space, which were caused by some fixed and variable factors that ultimately led to durability of Cairo as the capital of Shiite Fatimid caliphate, and fall and inefficiency of Fustat as a permanent urban, which was transmuted into a neighborhood of Cairo.