The library as a social agency must study and evaluate its effectiveness and seek to improve its service for achieving the external objectives for which it was created. The study is an attempt to define the determining factors involved in change in library service. Library service evolves as a consequence of social need, which comprises unconventional demands of emerging areas in the subject ratio of literature, change in methods of research, change in the educational level of the community, and change in the user behavior. Since the library is an agency of communication, growth and specialization of information, and increase in variety of information media have effects on library service. The library is one of the many communication agencies in society, and increase or decrease in their programs can be determining factors of change in library service. Today, libraries depend more and more upon interlibrary cooperation to allow them to overcome their limitation in resources and time, and they can bring about changes in their service by adjusting themselves to the interlibrary cooperation arrangements available. Finally, library service is rendered as a result of theorizing as to what the library might or should do, and naturally theories or change in them may be determining factors of library service.