Abstract: Ecoregions are regions designated on the basis of regional ecological equivalence and similarity in their potential productivity. The hierarchical classification of ecoregions that we call ecological division or ecological regionalization, is one form of natural regionalization.There are three ranks in the hierarchical system of ecological division unit:Ecoregion: Ecoregion is a region that shows a similarity in general not only in biotic climate but also in potential productivity and limited factors, and has the same dominant vegetation type on the zonal habitat. The whole region of Inner Mongolia is divided into seven ecoregions.Subecoregion: Subecoregions, on the other hand, are the natural regions among the ecoregion, which are characterized by greater similarities in biotic climate, landform and soil conditions as well as in utilization and management. These regions are indicated by vegetation subtype as well as its lifeform composition and synusia (or ecological species group) structure. The whole mongolian autonomous region has been divided into 35 subecoregions.Ecodistrict: Within subecoregion, the regional changes in the characteristics of the ecosystem are, taking place due to the vast land expanse and geographical separation, so each subecoregion may be divided into ecodistricts. The differences in ecological characters, potential productivity as well as utilization is much less than that between the different subecoregions.The details of ecological division in the Inner Mongolia Autonomous Region is shown in its ecoregion map.The sequential arrangement of these seven ecoregions reflects the regular change of nature, which is brought about by the fact that rainfall gradually decreases from east to west and temperature increases gradually from north to south. Besides, the following characteristics are reflected on this ecoregion map: 1. The overlap of horizontal zone and vertical zone is obvious. 2. Along the fringe area on the Inner Mongolia Plateau, the ecotone is quite evident. 3. The factors causing regional differentiation vary from place to place. In humid and semihumid areas of the eastern part, the dominant factors are temperature and rainfall. In the arid area of the western part, however, soil structure and topography become the dominant factors.The purpose of ecological regionalization is to offer us a better understanding of the characteristics of the land resources in the various ecoregions, so as to further improve our utilization and management of these resources.