Rainfall erosivity is defined as the power of raindrops to erosion, which can be expected to change in correspondence to changes in rain characteristics, particularly intensity and duration. This study was conducted to assess the effects of intensity and duration on rainfall erosivity in a semi-arid region in north west of Zanjan, Iran. Soil loss was measured at forty eight plots (60 cm Í 80 cm) under the simulated rainfalls with eight rainfall intensities and six durations at five replications in a sloped land (8%). Based on the results, out of 240 rainfall events, only 180 events caused soil loss at the plots. Rainfall intensity of 15 mm h^-1 was threshold intensity of the rainfalls from view point of soil erosion at the plots. Total rainfall energy of rainfall (E) obtained based on the sum of energies of all raindrops had a better correlation with the amount
of soil loss at the plots (r= 0.92). Soil loss strongly correlated with an index obtained from the multiplication of maximum-7.5 minute intensity (I_7.5) and rainfall energy (E), EI_7.5. About 96 percentage of soil loss at the plots could be explained by EI_7.5. The results indicated that, EI_7.5 as compared to the EI₃₀ can properly predict soil loss at the small plots similar to what usually happens in the surface erosion.