It is essential for generating DEM by interpolation from digital topographic datasets to determine an optimal cellsize, with which the basic terrain characteristics can be depicted by the resultant DEM without new errors to be induced. On basis of geomorphological theory, targeting to read all the information from topographic map data, including contours, spot heights and stream- line, and to describe the terrain features efficiently, the breakpoint from steep to flatten in the diagram of the RMS slope and resolutions could be regarded as an optimal resolution. The method was introduced and demonstrated with DEM interpolating based on 1∶250,000 and 1∶10,000 topographic datasets with contour intervals of 100 m and 5 m respectively in loess plateau. The results show that the resolutions for the two scales of topographic maps are 50 m and 2.5 m.