This study is to survey the present book numbers used in Korean libraries. to analyze their problems. and to suggest improvement in the book numbers. As the results of the survey several suggestions are derived as follows. 1. The education to the librarians about the kinds and the application methods of the book numbers used in Korean libraries should be sufficiently offered. 2. The present book numbers used should be expanded in detail for avoiding the duplication of the call numbers. Also when book numbers are assigned. shelf list cards should be examined one by one not to give the same number. If the book numbers are overlapped in the state not to expand further. libraries should establish a detailed expansion rules according to their respective situation and maintain it consistently. However, it is impossible for libraries operating open stack systems to arrange books on the shelves in call number sequence, therefore libraries can solve the problems as they accept duplication cases. 3. Since the object word of main mark IS a heading In main entry, we must apply cataloging rules to it closely. 4. For expanding book numbers widely it is desirable that the subsidiary mark of the book numbers which is the most prevalent in general and will be the most preferable In the future through the survey should be added and be provided. 5. The book numbers used being changed, we are to assign new book number leaving former materials as they are at the point of adopting a new book number. So aged materials are stored in a repository or on a compact shelf in the stack, will be discarded or weeded according to subjects after a certain period of time. 6. With library automation bringing together all the books of an author in a particular subject or in a particular form and avoiding completely duplication in open stack systems are meaningless. Rather than chronological book numbers, distinguishing clearly new materials from aged materials and controlling the stack spaces mechanically and effectively, can be regarded as the most modern and future-oriented of all the book numbers.