GIS is a very useful tool in modelling the earth surface and defining basin parameters to be used in topography-based hydrological modelling. Grid based Digital Elevation Model (DEM) is used for modelling topography and grid size is the main parameter that will affect the accuracy of DEM directly and hydrological modelling indirectly. The objective of this study was to examine the effect of grid size and map scale on geomorphological basin parameters. The study was conducted on three small research basins in central Anatolia, Turkey, for which these parameters were determined and analysed with different grid sizes and different map scales. The grid operations: surface modelling, flow direction, sinks, filling sinks, flow accumulation, basin delineation, stream network, ordering and vector conversion were made with a GRID module of Are/Info software. It was found that as grid size decreases more detail can be observed, but this is true for steeper areas. Also, as the basin area increases, the effect of grid size increases. Considering the map scale, grid size selection is a tradeoff between the accuracy needed and computation time.