Universal Soil Loss Equation (USLE) model and GIS were used to evaluate soil erosion in Longdun Reservoir watershed of Nanjing. The results showed that Longdun Reservoir watershed was moderately eroded, and the average soil erosion modulus was 4 343.46 t/km2. More than 63% of the watershed area was at the level of micro or slight erosion. The watershed area with severe erosion was very small, but it contributed to more than 70% of soil erosion amount. Soil erosion in this watershed varied with land use types, and the annual average soil erosion modulus followed the order: upland field > grass > paddy field > forest land, and the soil erosion amount followed the order: upland field > paddy field > grass > forest land. The watershed was divided into 13 sub-watersheds by GIS. The sub-watersheds 4, 5 and 10 were the areas with large soil erosion modulus, and the more attentions need to be paid to the sub-watershed 10 because of its large soil erosion modulus and soil erosion amount. The sub-watersheds 1, 9 and 12 should also be taken care of due to their large soil erosion amounts. Soil erosion amount in all sub-watersheds mainly came from the areas with high erosion grades and severe erosion. Therefore, the more attentions should be paid to the areas with high erosion potentials when the control measures are taken to inhibit soil erosion in future.