Abstract: Constitutional democracy has been the basic form of political system in western countries in modern time.As reflected in history，constitutionalism and democracy both have close connection with “rule of law”.The concept of “rule of law” started in ancient Greece which emphasized that no person is above the law，and democracy was a natural fruit of such concept and took the form of direct democracy in ancient Greece，the extreme of which was the French Revolution.But direct democracy led to the lack of order and thus the lack of rule of law in the end.Afterwards，democracy and rule of law reached a balance in the process of evolution.The mile stone of British evolution of rule of law，the Great Charter，established the early framework of constitutionalism.Modern and contemporary democracy and rule of law，represented by constitutionalism in the United States，mostly perform within this framework.

Key words: rule of law, constitutionalism, democracy, written constitution, unwritten constitution