The purpose of this study is to analyze characteristics of apartment housing unit plans based on analyzing overall patterns of housing unit plans and identifying the types of unit plans according to the size and to provide the fundamental data for developing diverse dwelling unit plans of apartment. This study deals with apartment housing unit plans built between 1999 and 2000 in Seoul, the capital region and new town. The major findings of this study are as follows; 1) Most apartment housing unit plans have fixed and uniform layouts. Most apartment housing units have L-DK type meaning they have isolated living room space. Certain layouts(especially $60-85 extrm{m}^2$ sized unit plans) show a "trickle-down" phenomenon with respect to the areas of units according to time series. 2) New plan types are rarely proposed since 2000, especially in large-sized(area more than $120 extrm{m}^2$) and $60-85 extrm{m}^2$ sized plans. 3) Another feature of apartment housing unit plans is increase of the service are (balcony, entrance area, additional kitchen, and storage area), and introduction of masters space (composition of master bedroom, dressing room, powder room, path, and etc.) and etc.)