This study focuses on "Intellectual Freedom" based on the fundamental principles of library services. Particularly, this paper pursues to the legal foundations of "the freedom of expression". The American Library Association celebrates "the Banned Books Week" to draw attentions to the danger of banned books and censorship. This study also studies how "the banned books" are made by pursuing the definitions of "pornography" and "obscenity" in terms of the court's opinions of The Supreme Court of the United States and Korea. Recently, the top three reasons for the "banned books" are considered to "be sexually explicit", "contain offensive language" and "be unsuitable to age groups".