Under erosion conditions, velocities and sediment concentrations in water flows running down five slopes different in gradient were measured.Results show that sediment concentration changes with velocity.The former is positively correlated to rill development.At the beginning of rill development, velocity of the flow increases gradually, and when rills come basically into shape, it decreases and then levels off.The velocity of the flow is the main factor causing production and transference of sediment.When the gradient of a slope is low, rills develop slowly and little sediment is produced.The production of sediment does not differ much when the slope varies between 10°~20°.When the gradient is at 25°, however, even less sediment is produced than 20°indicat ing a critical gradient existing between 20°and 25°.Based on the characteristics of a slope, analysis of the relationship between velocity of the flow and sediment concentration therein may help explain reasonably the critical gradient of the slope.When the gradient of a slope is bigger than the critical value, velocity of the flow increases at the beginning, bringing down sediment not less than that on the slope with gradient smaller than the critical value.In this case, a deep sink is more likely to form, thus reducing the velocity of the flow.Rills, however, complete their development within a short period of time, causing the velocity of the flow and sediment concentration therein to decline.As a result, the cumulated sediment eroded within a certain time from the slope with gradient bigger than the critical value is less.The measurement of velocity with the electrolyte pulse method explains some phenomena of the erosion process.If the spatio-temporal distribution of velocity is further measured, the study may likely go deep enough to reveal mechanism of erosion and provide more accurate parameters for establishment of a rational model for prediction of soil erosion.