Soil erodibility is one of six factors in the revised universal soil loss equation (RUSLE) that reflects the ease with which the soil is detached by splash during rainfall and/or by surface flow. This study was therefore carried out to assess applicability of the RUSLE model in estimating erodibility factor (K) and develop an appropriate equation to predict this factor in soils of the semi-arid region in Iran. Thirty six dry-farming lands were considered in a 900 km(2) agricultural zone in Hashtroud, northwest of the country. Soil loss was measured at 108 unit plots under natural rainfall events for a 2-year period from March 2005 to March 2007. The K-factor was estimated using the mean geometric diameter of soil particles (D-g) and measured based on the mean annual soil loss per unit rainfall-runoff erosivity factor (R). Based on the results, the estimated K values varied from 0.0316 and 0.0485 t h MJ(-1) mm(-1) and the measured K values were ranged from 0.0014 to 0.0050 t h MJ(-1) mm(-1). The measured K values were almost 14 fold smaller than the estimated values on average. There was no considerable correlation between the measured K-factor and Dg (R-2 = 0.05). Multi-regression analysis showed that the measured K-factor was significantly related to the aggregate stability and permeability (R-2 = 0.90, p < 0.001). Although D-g was positively correlated with the soil permeability (R-2 = 0.42), it did not strongly affect the measured K-factor (R-2 = 0.05) because of its negative effect on aggregate stability (R-2 = 0.64)