No such thing. Rome was a Republic at that time. Gaius Julius Caesar had been acclaimed Imperator (Emperor) by his troops, but at the time this was a military title, equivalent to Supreme Commander. He ruled Rome as Dictator, a title he assumed after his defeat of Pompey and forces loyal to the Senate in 45 BCE.

The first true Emperor was Gaius Octavian Caesar, Julius' great-nephew and adopted heir. Octavian, inherited Julius' assets - including military forces -after Julius' assassination. He was, along with Marcus Antonius (Mark Antony) and Marcus Lepidus, one of the Triumvirate who defeated Cassius' faction and assumed rule of the Empire. Octavian was almost as adept as Julius in boith politics and warfare, managing to get Lepidus exiled and defeating Mark Antony and Queen Cleopatras' forces. Thus by 27 BCE. Octavian was undisputed ruler of the Empire, later to be known as 'Augustus' (the Great).