This study aims to analyze the projects of Rem Koolhaas and Kazuyo Sejima in terms of program organization and spatialization methods. Diagrammatic methods are frequently used in their design process and are articulated in the graphical presentations in many publications to explain their projects. Even though diagrammatic methods can be used in diverse ways, Koolhaas and Sejima tend to use them to systematically organize the given programs in their specific ways. Furthermore this organization generate spatialization process in which the final building form is flexibly derived from the process itself. After the detailed studies for each architectsquot concepts, design process, and several unique characteristics that have vital roles to construct their ideas into real building were found. This research has a couple of steps to critically analyze the spatialization process: Conceptual Background, Circulation Organization, Structural Method and Furniture. After the examination based on these elements, similarity and difference between two architects are discussed in the conclusion. Ultimately it has been found that there are unique design development processes in each architectquots projects even though they tend to apply somewhat similar program-based spatialization process. It has originated not only from the each architectquots personal view of architecture, but also from the cultural backgrounds. It is hoped that this sort of comparative study would generate more diverse discussions in program-based design process as well as clear application of any related methods.