Soil loss is estimated by different models in which the soil erodibility factor, K, is one of the important parameters, especially in soils with rock fragments. The objective of this study was to determine the soil erodibility factor by the Universal Soil Loss Equation (USLE) and USLE-M models by direct soil loss measurements for six soil series (three of them contained high amounts of rock fragments) under a rainfall simulator with rainfall intensities of 26–55 mm h−1. The erodibility factor of the six soil series varied between 0.0053 and 0.0125 t ha h (ha MJ mm)−1, in which the three gravelly soils (Bamoo, Loamy-skeletal over fragmental, carbonatic, mesic, Typic Xerorthents; Kuye-asateed, Loamy-skeletal over fragmental, carbonatic, mesic, Typic Xerorthents; and Shekarbany, Fragmental, mixed, mesic, Typic Xerorthents) had medium K values [0.006 t ha h (ha MJ mm)−1]. Other soils (Pump-namazi, Fine, mixed, mesic, Fluventic Haploxerepts; Ramjerdi, Fine, mixed, mesic, Fluventic Haploxerepts; and Daneshkadeh...