The purpose of this study was to investigate the current status of assigning book numbers and the perceptions of librarians who assign the book numbers at two National Libraries, nine Regional Central Libraries and Jeongdok Public Library in Korea. Based upon the analysis, a better method to assign book numbers for the libraries was suggested. The most serious problem was the length of the book numbers due to the duplication of book numbers and lack of intuitiveness. Therefore, this study suggested a user-oriented way of assigning book numbers based upon combining the name of the author and the year of the publication.