Field and laboratory model experiments were carried out to evaluate the K factor of brown soils (Orthic luvisols) developed from loess in Lublin Upland (SE Poland) according to RUSLE. The soils represented different erosion classes. The K-values calculated from the nomograph appeared to be very high (0.055-0.066), which suggest high susceptibility of the soils studied to erosion. Contrary to this, initial results from field studies carried on according to USLE standard show that the K-values for all soils are more than 10-fold overestimated. Especially high discrepancy between the values of soil losses estimated according to RUSLE and those obtained in the field occur in the spring/summer period, characterized by high EI values. Nevertheless, the K-values reflect in a proper way the differences in erodibility among the studied soils. That was confirmed by soil losses recorded in the field and laboratory experiments with the use of rainfall simulator.