Cross-Over as an aesthetic phenomenon is closely associated with postmodernism. So we have to return once again to the relation between modernism and postmodernism. Postmodemism especially relating to cross-over is defined as anti-modernism. Postmodernism as antimodemism considers modernism as too intelligent, difficult, elitisic art. Moreover modernism is no longer an actual artistic theory but just a past theory. Elitism of modernism is replaced by populism of postmodemism. Especially after 1980's, Crossover, characteristic of postmodernism as being oriented towards popular culture has been seen in every apect of life. For example, crossover between high culture and popular culture, between history and fiction, even between image and reality. Postmodemism is a principal logic in society where every hierarchy is demolished and an aesthetic version of democracy, pluralism and relativism. In this essay, Postmodernsim relating to crossover is regarde and interpreted as an American cultural theory. So it is a very serious problem that Postmodemism as a national and local theory is accepted as if it is a global cultural theory. The call for modernism in the middle of 19th century was in connection with the question of the status of art in the capitalism. In the capitalist society where every human action and meaning was reduced to money, modernism chose to isolate itself from contemporary society and dedicated itself to art. It limited its main question to the artistic experiment. The Art Cult of Modernism was a resistent activity of modem artists against their society. In that process, art inevitably became esoteric, ambiguous and difficult. Because of its difficulty, thus, Modernism is accessible only to a few spectators. In that situation, numerous constructions of museums and concert halls for industrial and financial aim made Modernism much expensive commodity. The others of Modernism, for example, female artists, homosexual artists and artists of colored races criticized occidental, white, male centrality of Modernism. They argued Modernism is not a universal but only a local aesthetic theory. Postmodemism as one of the others of Modernism appears in criticizing those negative aspects of Modernism. It is argued that Postmodemism is not only a new cultural and aesthetic logic but also a new historical theory. But the reason why this essay interprets postmodernism as an American theory or a kind of americanism lies in the fact that the program of postmodernism itself is solely American. In America as multicultural nation, there can be various heterogeneous cultures, and even they can coexist peacefully. America has no dominant high culture handed down fiom old times. American culture is represented as popular culture. So Postmodernism as affirming lightness, joke, style and appearance instead of heaviness, reality and meaning can be propagated in America. It is natural that Postmodemism is an American theory. But it is not a local theory. Postmodemism as Americanism or cultural imperialism wants to homogenize cultures of entire world and at last dominate that world mentally. Postmodernism is cultural logic of political dominance of all the world by America. In conclusion, we must call crossover in question instead of treating it solely as an aesthetic phenomenon.