Erosion of water contributes to soil degradation and siltation of rivers and water reservoirs. Identification of areas prone to erosion is obtained using mathematical modelling in conjunction with geoprocessing techniques. Therefore, the aim of this study was to analyze rainfall distribution and to estimate the erosiveness factor of rain for Ribeira Valley, Sao Paulo State, Brazil. To this end, rainfall data, the R Factor from Universal Soil Loss Equation (USLE), and interpolation process by the Inverse Distance Weighting (IDW) method were used. The values obtained for rain erosivity have demonstrated a high variability of the erosive potential with amplitude from 5,360.6 MJ.mm.h(-1).ha(-1) to 9,278.7 MJ.mm.h(-1).ha(-1). Areas with greater erosivity potential of rainfall were the North and Northeast regions of Ribeira Valley and, consequently, are the most vulnerable to anthropic interventions.