The hydrodynamic characters of runoff in the process of rill erosion were studied by using runoff scouring experiment under the different slope gradients and the different flow discharges. The results show that the reynolds number of the flow varies between 362 and 4 282 in the erosion process, based on the various of reynolds number the erosion process can be divided the sheet erosion stage and the rill erosion stage. The Froude number of runoff flow larger than 1 under the all experiments and it is belongs to the turbulent condition. Roughness coefficients in rills that are easily affected by the slope gradient, are closely related to reynolds number. That is, the roughness coefficient decreased with the increase of reynolds on steep slope. Flow velocity showed a little increase after a decrease with experiment time length of scouring and the variation of soil bed irregular in profile and cross section.