The article is devoted to analysis of the US Constitution as the main source of American law. Such main principles of the American constitutionalism as the separation of powers and federalism are observed. The author defines relation between the constitutional enactments and precedents. It concludes that U.S. Supreme Court owing to its caselaw based on constitutional provisions per se makes new constitutional rules. In the article specific features of constitutional judicial review and its influence on the American Constitution are also characterized. It is underlined that owing to the existence of the United States Constitution as a written document with the supreme force the American legal system occupies the specific place among common law countries.