Abstract: This paper presents a method to subdivide and code a large river basin with the aid of GIS techniques.By overlapping the surveyed watercourses on the DEM,the elevation values of some convex cells in rivers are modified to remove the large-size falsely-closed depressions.Then a river network consistent with the surveyed river network is extracted from the modified DEM.The extracted river network is coded prior to subdivision and codification of the river basin.This paper also brings forward the improvement to the Pfafstetter coding rule because the area sizes of sub-basins at the lowest level may be quite uneven if the rule is directly adopted.The river network and the basin are coded according to the improved Pfafstetter coding rule.All tasks of codification are automatically accomplished by using a program developed in this study.The suggested method is applied to the Yellow River basin and the whole drainage basin is divided into 8255 sub-basins,of which each is assigned with a Pfafstetter code.