Many reservoirs in Taiwan were built to retain water for distribution during dry seasons. To increase the design life of reservoirs, it is important to assess the annual sediment input to reservoirs. This study used one of the most important reservoirs in Taiwan, the Shihmen Reservoir, as an example to estimate the amount of sediment derived from soil erosion of the surrounding watershed using the Universal Soil Loss Equation (USLE). There are five factors of USLE, namely the rainfall factor Rm, the soil erodibility factor Km, the topographic factor LS, the crop management factor C, and the erosion control practice factor P. Values of Rm and Km were obtained from various published literature. For topographic factors, the ASTER Global Digital Elevation Model (GDEM) was used to compute the slope length and the slope gradient. Finally, the C factor was obtained from the land use/land cover data of the National Land Surveying and Mapping Center (NLSC). The result is a spatial distribution map of soil loss in the Shihmen Reservoir watershed.