The purpose of this research was to study the structure and the use of three Thai author numbers tables devised by the Central Library, Chulalongkorn University; the National Library; Thamasat University Library with the hypotheses that: author numbers have a difference in their structure and use, and the suitable Thai author numbers to be applied with Library of Congress Classification System should have the same structure and use with the Library of Congress cutter numbers. The research was conducted by gathering data from printed materials, library card catalog, 25 questionnaires distributed to librarians of six central university libraries that use Library of Congress Classification System, and interviewing of 7 catalogers. The summarized findings are as follows: The Central library of 6 universities have the same policy in assigning close call number to create efficiency in book accessing. Most of the library have over 20,000 volumes and separate their technical work for Thai and foreign library materials. The majority of librarians have the Bachelor of Arts degree in Library Science plus 4 to 6 years of library experience. Their skill in classification are obtained from library work, and classification course provides by library schools. Usually in assigning a call number, the Library of Congress Classification scheme as well as the library shelf-list card are consulted. The data from questionnaires supported the documentary study that Thai author numbers of Central Library, Chulalongkorn University and the National Library contain important elements of Cutter-sanborn three-figure Table and the Thai author numbers of Thamasat University has the same structure with the Library of Congress cutter numbers. The study also reveals that this author number table is popular among catalogers because of its flexibility and easy assignment of one or two figures to initial letter of the author's name. Recommendation: 1. In applying a Thai author number table, a librarian should carefully study its use and library's policy of technical work. 2. A library should have a manual in assigning the symbols by which names are differentiated in the call number. 3. A shelf-list card should be consulted in assigning a call number. 4. A library with less than 20,000 volumes may use only an author's and title's initial letters for a work mark. 5. A library should arrange an in-service-training in Library of Congress Classification System for a new staff members.