This article proposes to pay attention on the sources of historical demography in the old eparchy of Arad. At the end of XVIIth century, the Ottoman domination in the zone have been filled with the domination of Habsburgs and the Arad city became a significant orthodox Episcopal centre subordinated in religious plan to the metropolitan of Karlovitz. For the reconstitution and the knowledge of the demographical aspects into old counties Arad and Zarand, we can use many documents, which are preserved in the Departmental Direction of National Archive of Arad. Various censuses, tax conscriptions, wills and especially parochial registers of the marital status are sources of first category. With the methods used by Michel Fleury and Louis Henry this sources are essential to know the behaviours demographic for the various ethnos groups, as well majority Rumanian as the population German, Hungarian, Serb or Jewish which have lived in these places 300 years ago.