A strong earthquake can trigger thousands of co-seismic landslides. The evaluation of the volume of such co-seismic landslides is essential for landslide risk assessment and long-term post-earthquake hazard evaluation. Empirical volume-area power laws are often applied to estimate the landslide volume. When several small landslide deposits form a large cluster on the hillslope, blurry boundaries of the clustered co-seismic landslides may lead to seemingly one large landslide and grossly overestimated landslide volume. In this paper, we propose a flow direction-based method to identify the boundaries of a cluster of landslides and to partition co-seismic landslides. Three watersheds that were severely impacted by co-seismic landslides in the Wenchuan earthquake region are chosen for case studies. Each co-seismic landslide cluster in these watersheds was interpreted from images taken shortly after the earthquake, and partitioned into individual landslides in accordance with a flow direction map that is processed based on a local digital terrain model. Finally, the volume of each individual landslide is calculated using an empirical volume-area power law. The adopted power law scaling relationship is selected from six widely used empirical relations and verified by 11 local landslide cases. The proposed flow-direction based partitioning method is further verified by clear boundaries of landslides that were exposed after storms, as well as supplementary field survey results. The proposed method results in similar landslide boundaries and volumes as the field survey results. The uncertainty of this approach is below 19%. If no partitioning is conducted, the total volume of the landslide cluster can approach twice that from the field investigations.