The present paper deals with the history of military corps, which Roman emperor Constantius II sent to Great Armenia in 338/339 A.D. for to fight with the Persians. The main evidence on Constantius II’s Persian expedition are preserved in the works of prominent Armenian historians, that are Moses Khorenatsi and Pawstos Buzand. Constantius II held this campaign in order to free Armenia from the Persians and to bring back its state freedom. According to Moses Khorenatsi and Pawstos Buzand, Constantius II sent in Armenia bodyguards, who were representatives of Armenian noble families allied to the Romans, but the Roman regular forces themselves collected in Galatia (the Asia Minor), moved in Mesopotamia and Atropatene (south of modern Azerbaij...