For Chinese history,"emperor" is more proper.It meas 皇帝，and "King" means 国王。

A king usually refers to the ruler of a single country or area and an emperor is the ruler of a number of countries. But, at that moment those countries aren't seen as countries anymore but parts of an empire. Examples are The Roman Empire and the Holy Roman Empire. (Although the HRE didn't even include Rome.) An emperor is more powerful than a king because there was usually just one emperor in Europe. (Charles V, Wilhelm II, Frans Jozef I, etc.) But the emperors were usually king of one of the members of the empire as well. Just look at Charles V list of traits: Holy Roman Emperor, King of Aragon, King of Castile and Léon, Archduke of Austria, Titular Duke of Burgundy, Ruler of the Burgundian Circle For China we always use Emperor. (And for Russia we use Tsar. (Tsar is a tital derived from the most famous Roman ruler: Caesar))