Hagia Sophia, is the first large church of Christian universe. After the conquest of Istanbul by the Ottoman Empire, it had been used as mosque and it has kept its value among all important mosque of Islam universe. This church which is still standing as hard as nails today, have been started to be constructed by the command of Emperor I. Justinianus in 532 B.C. and its construction had been completed on the date of 17 December 537 and then it has been opened for religious worship activities. Thousands of workers have been occupied during its occupation. Hagia Sophia has an area of 7570 m2 and its dome containing 40 windows has an altitude of 55 m from the base. The weight of the dome has been distributed over to other columns by using four feet called as filpaye. The mosaics that have covering the walls and ceiling of the church has an area of 1600 m2 nevertheless most of the mosaics have been poured out and lost from the past untill today. On the date of 29 May 1453 after the conquest of the city by Turks, Fatih Sultan Mehmet the Ottoman prince had commanded the transformation of the church into a mosque, then the church had been immediately put into service as a mosque. Because of the high respect and faith of the Ottomans to other two great Religions the mosaics have never been pulled out from its original places. In contrast, they have been covered them by plastering with lime Therefore, the mosaics of Hagia Sophia has been maintained for passing centuries. “Allah” and “Muhammed” names are read on two big circular panels which stand on two sides of Absid part. Omer, Osman, Ebu-bekir and Ali names which are the Four Caliphs have been written on other circular panels. On the upper gallery of the Church; namely on first flat of the church, there are some compositions that show the famous Empress Zoe who had shifted three husbands and her third mate constantin Monamakos; and also show Emperor II. Jean Comnenos and his wife Irene. In addition to that, on again the same flat worldwide famous scene called as “Deisis” or “Dua” which have been composed of the mosaics of Jesus, the Virgin Mary and Baptizer Yahya, is one at the most important places that have been to be visited. The building located on a corner of the garden of the church contains the tombs of Sultan II. Selim, III. Mehmet, III. Murat and their wifes and their children and it has been once used as the baptistery part of the church. Hagia Sophia, which is the crowned building and has the highest prestige among the buildings in Istanbul, has been transformed into a museum by the commands of Ataturk in the year of 1934.