Library Cooperation is a trend of modern librarianship. The union catalog has been a foundation of library cooperation and an indispensable tool of bibliographic control: It also plays a decisive role in the effective execution on interlibrary loan and resources sharing. The author described the history and development of the union catalog in the R.O.C. with using statistical tables and graphics to analyze the feature of union catalog, introduced some union catalogs about Chinese studies published in foreign countries, and tried to discuss the problems and explore some possible solutions.