The objective was to make a water erosion vulnerability map to the scale 1:25,000, by geoprocessing USLE, in order to identify erosion threatened areas and help the future planning of land use and environmental management of the drainage basin. The watershed is located on the rural zone of Angra dos Reis, RJ (22°50'19"/22°55'52" S and 44°13'53"/44°19'41" W). It's in the domain of Serra do Mar, presenting rough topography, with gradients ranging from sea level to 1246m, in an area of 68km². The lithology is constituted of Precambrian granites and gneisss and Quaternary fluviomarine deposits. Cambisols (17% of the area) occupy the greatest altitudes and slopes, Acrisols (31%) are in the base hillslopes and in basal hills and the fluvisols (16%) ocuppy the plain. The mean annual rainfall is 2,100mm, with more than 200mm monthly values. The drainage length totalizes 139km. The Tropical Rain Forest covers 61% of the area, mainly in the highest and steepest hills. Other land cover and economic activities are banana plantations, cattle ranches, sand and stone quarries. When applying USLE in a GIS, the product of soil erodibility and topography maps with the rainfall erosivity factor, allowed assess the Natural Erosion Potential, which can be very high in 22% of the basin. The most threatened sites are the ones that have litholic soils, mountainous relief, slope lenght above 100m, slope steepness greater than 50%, and altitudes greater than 400m. After introducing the land use/land cover map, 51% of the area yields less than 1 t.ha^-1.yr^-1. Comparing the Actual Erosion map and Tolerable Losses allowed identification of critical sites where the actual erosion is larger than the tolerable limit (36% of the area). With the help of ancilary data available from the local authorities it was possible to recommend a more sustainable management plan for the watershed.