Purpose: The modern library is transforming from a national public institution to a regional space due to the expansion of local self-government, and accordingly, there is a trend of diversification such as a small library and a library as a place for information exchange. From this perspective, this study aims to understand the spatial configuration and transitional space concept of a modern library based on theoretical considerations in a changing environment, categorize spatial characteristics and composition methods, and present functions and compositional effects thereby. Method: Through literature review and theoretical analysis, the concept of transitional space of a modern library is derived, its characteristics are classified, and the transition in the formation of its spatial order is considered. The concept of transitional space, spatial configuration, and function are analyzed through the case of a representative library in a local large city in Korea, and based on this, types for universal application in library space arrangement are included. Results: First, when the entry space of the library is analyzed as a transitional space, its spatial characteristics can be analyzed by dividing it into a new space, a dichotomized space, and an intermediate space by the program of its interior space connected to the transitional space. Second, there are four functions of the transitional space of the library, which can be analyzed in terms of connectivity with the city, affordability of interior space, affordability of exterior space, and physico-psychological accessibility.