The paper deals with the problem of development of the Late Roman military titles, the issue under consideration is the titles of magister equitum («a master of cavalry») and magister equitum et peditum («a master of cavalry and infantry»). These titles were in the hands of generals, who held an office of master of soldiers (magister militum), such generalswere the supreme commanders of all Late Roman expeditionary troops. The author turned to and considered the evidence of some sources, that are a law of Constantius II published in AD 347 and the chronicles of Late Roman and Byzantine historians (Aurelius Victor, Zosimus, Iohannes Zonaras). Having compared the evidence of sources enumerated above, the author concluded that Constantine I the Great, who established the offices of masters of soldiers, appointed two these generals, but, in contrast to previous scholarship, the author supposes that each of these masters commanded both cavalry and infantry at the same time. The previous scholarship argued that Constantine I established a separate office for cavalry and another one for infantry, according to this widespread opinion, each of the masters commanded only over cavalry or infantry, but not over two branches of troops together. The author concludes that the title of magister equitum mentioned in a law of AD 347 really was just an abbreviated version of the full official title magister equitum et peditum («a master of cavalry and infantry»), therefore, as the author supposes, each of the masters of soldiers, who existed during AD 337-350, held a title of magister equitum et peditum, i.e. he had a combined military command. The full separation of supreme command and an establishment of two single offices, that are a master of infantry and a master of cavalry, happened only after AD 350.