The Roman Emperor Constantine I moved the capital of the Roman Empire to the city of Byzantium , and he renamed the city Constantinople . Centered on the capital of Constantinople , it was ruled by emperors in direct succession to the ancient Roman emperors after the collapse of the Western Roman Empire . As the distinction between `` Roman Empire '' and `` Byzantine Empire '' is largely a modern convention , it is not possible to assign a date of separation , but an important point is Emperor Constantine I 's transfer in 324 of the capital from Nicomedia ( in Anatolia ) to Byzantium on the Bosphorus , which became Constantinople , `` City of Constantine '' ( alternatively `` New Rome '' ) .