Recently, Digital Elevation Models (DEMs) can be often used in the analysis of catchment topography and hydrological process. Some methods that extract drainage networks from square-grid DEMs are proposed. They make use of the flow-accumulation-value threshold for extracting drainage networks from delineated overland flow paths, but the relationship between chanel length, L, and catchment area, A, of the external link in the drainage network from DEMs is different from that of actual drainage networks. This paper proposes a new threshold, which is defined by the ratio of the width, w, to the penetrated length, l, in a contour line.As a result, it shows that this relation is improved. Furthermore, we examine the hydrologic response of both drainage networks, and represent effects of the threshold for this response.