The purpose of this study is to determine the high accuracy boundaries of Western Mediterranean Basin. In accordance with this purpose, a digital elevation model with 10 m horizontal resolution is generated and the basin boundaries are derived from this model through the tools of geographical information systems using the high accuracy 3D vector elevation and hydrography data at 1:25,000 scale. One of the sub basins at first level is a closed sub-basin, which is derived by using a sink in the model corresponding to Avlan Lake. It does not exist in the current basin sets. The accuracy of basin boundaries and digital elevation model are evaluated with respect to the weighted root mean square errors in position and height respectively. Consequently, it can be stated that basin boundaries and digital elevation model are obtained at high accuracy.