The article is devoted to the problem of legalization of usurpers’ power in Byzantium in IV-VI centuries. Action which usurpers took to fight for power was presented in the first part. As a rule pretenders to the throne relied on the army, they also tried to gain a support of inhabitans of Constantinople if the usurpation took place in the capital or of people of that part of the country where the riot started. The elements of legalization process of usurpers’ power are analysed in the second part. The usurpers tried to fulfil the requirements of the election procedure and they were: proclamation by the army, recognition by the senate and people of Constantinople, coronation. It was the model of electing the emperor in the period of interregnum. Occasionally the acceptance of the legal ruler for own promotion was tried to be obtained what, in the light of the constitutional practice, should be the essential element of the process of legalization of power. Sometimes the person of the empress was used for that aim as in the case of usurpation of Leontius. When they didn’t want or couldn’t obtain the emperor’s acceptance for their own promotion they had to defeat him in a fight. The victory caused that the usurper became a legal ruler still after fulfilling the formal requirements which he couldn’t do earlier. Zadanie pt. Digitalizacja i udostępnienie w Cyfrowym Repozytorium Uniwersytetu Łódzkiego kolekcji czasopism naukowych wydawanych przez Uniwersytet Łódzki nr 885/P-DUN/2014 zostało dofinansowane ze środków MNiSW w ramach działalności upowszechniającej naukę.