During the 4th century a central government of the Later Roman Empire usually attracted different external tribes to Roman military service, with respect to Western provinces this policy inevitably was applied to German tribes, which lived at the Roman borders across the Rhine. Meanwhile, the author wants to clear, how a roman government created regular military units both from external and internal ethnic groups, i.e. units, from which expeditionary and frontier armies composed of. The author traces peculiarities of internal organizational structure of such the units and a legal condition of the tribal aristocracy, who leaded these ethnic groups. In order to study the question the author turned to hagiography (“Life of Saint Martin of Tour” and “Life of Ambrosius of Milane”), classical narrative accounts (Ammianus Marcellinus and Zosimus), and also to inscription from the city of Strumica (Republic of Macedonia). The author concluded that an internal structure of such ethnic units was a similar to another regular units and that ethnic units themselves were headed by their tribal chiefs, these chiefs received the titles of tribunus and protector.