The title of `` emperor '' was passed from the Romans to the Frankish kingdom when , on 25 December 800 , Pope Leo III crowned Charlemagne , king of the Franks , emperor of the Holy Roman Empire .
The standard designation of the Holy Roman Emperor was `` August Emperor of the Romans '' ( Romanorum Imperator Augustus ) . When Charlemagne was crowned in 800 , his was styled as `` most serene Augustus , crowned by God , great and pacific emperor , governing the Roman Empire , '' thus constituting the elements of `` Holy '' and `` Roman '' in the imperial title .