Land cultivated in paddy increased almost ten-fold in Northeast Thailand during the twentieth century and now accounts for almost 40％ of the area of this region. Such intensive paddy expansion was attained through reclamation not only of natural wet or irrigable land, but also of somewhat dry, hilly land. It is considered that the increase in dry paddy fields in hilly areas has caused land degradation, by reducing the stability of land, and decreasing rice yields in the area. This paper provides a background to the processes involved in paddy expansion in the Thap Than River basin in the southern part of Northeast Thailand based on the results of interviews with local people and an analysis of geographical information. The results show that lower-lying lands are not always reclaimed earlier, because the migration of people to land suitable for rice cultivation is often restricted by the lack of a road network. The percentage of irrigable paddy area to total paddy area has not changed significantly except in the Sangkha District at the origin of the Thap Than River system, where rice land has been developed in areas in which the riverbed slopes relatively steeply. Only in this district has rice production drastically shifted from irrigation-based to rain-fed.