Quantifying critical conditions of rill formation can be useful for a better understanding of soil erosion processes. Current studies lack a consensus and related rationale on how to describe these conditions. This study was based on the concepts that (1) the shear stress available for erosion at any given point is a function of the runoff rate, the slope steepness, and hydraulic characteristics of the surface; (2) rill incision begins when overland flow shear stress exceeds soil critical shear stress; and (3) the distance from the top of the slope to the point where rills form can be measured and analyzed as length to rill initiation and decreases with increase in slope and rainfall intensity. These concepts were tested with a representative silty-clay soil from the Loess Plateau in northwestern China on a large sloping indoor plot (8 x 3 m), with five different slopes using simulated rainfall at three rainfall intensities. Values of several hydraulic parameters at rill initiation were determined from the experimental data. The results showed relationships among slope steepness, rainfall intensity, and location of rill initiation. It was found that slope was relatively more important than rainfall intensity in determining the location of rill initiation. Soil critical shear stress determined in this study ranged from 1.33 to 2.63 Pa, with an average of 1.94 Pa. Soil critical shear stress was inversely related to slope and was not influenced by rainfall intensity. The results of this study were comparable with those of previous investigators.