In order to understand the runoff-sediment yielding processes of rill erosion for different soils, the characteristics of runoff and sediment yield in rill development for Lou soil (silty clay loam) and loess soil were studied under different slope gradients and simulated rainfall conditions. Results show that the runoff generation occurred earlier and rills formed more easily within Lou soil than within loess soil. The rill formation exhibited a regular parallel distribution. The degree of rill development increased as the gradient increased, which could prolong the time of runoff generation. For loess soil, however, it was hard to generate runoff and induce rill formation. The rills that did form exhibited a random orientation. The processes of rill development did not have a significant effect on runoff yield; the soil infiltration rate was the major factor causing the difference in the runoff generation processes in the two soils. For Lou soil, the rate of sediment production increased with an increase in the degree of rill development. Sediment production stabilized early, and sediment concentration increased as rill density increased. For loess soil, the sediment concentration process largely depended upon the randomness in the rill development process. When the rill erosion was based on collapse, a sharp increase was noted in sediment concentration, where even low rainfall intensities could cause significant soil erosion.