Critical conditions for rill flow erosion on hillslopes are investigated using numerical simulation and laboratory data for large variation in slope gradients. Several factors have been considered in this study, i.e., slope length, gradient, projected length on a horizontal plane, etc. Shear velocity is used as the parameter for establishing a critical condition, and two cases have been considered,i.e.,(1) the critical shear velocity may change with slope gradient, or (2) it may not change. For given plot size, net rainfall intensity and original topography, the minimum slope gradient has been found out at which shear velocity of rill flow can achieve the critical value.