Marketing is the business operation that many different companies or organizations use to promote their products or enhance their image to their customers or users. Then it derived different marketing practices and models for different situations. Target marketing is one of the most effective and popular choices. It is a kind of customer marketing, mainly using analysis of customer characteristics to segment customers, in order to achieve specific commodity marketing for the purpose. Library, especially public library, how to borrow the concept of target marketing to promote its utilization and services is a problem that library managers must actively conceives. The Pareto Principle is currently the most important and popular management rule applied to marketing, customer relationship management, services or products promotion, etc. This study will explore whether there this principle exists for patrons’ borrowing in public library, thus as the basis of target marketing. And we will apply data mining technology to analyze the characteristics of the vital patrons, in order to further serve as references for setting target marketing strategy. This paper was the first research to reveal that the Pareto Principle could be found in circulation data of public libraries in Taiwan. The results can help libraries identify vital patrons and major collections, and improve the efficiency of their management and marketing in future.