Using a typical silt clay soil from the Loess Plateau, laboratory simulated experiments were conducted with a flume on 5 different slopes(5°,10°,15°,20°,25°) and 3 flow rates (2 L/min,4 L/min,8 L/min) to determine the relation of sediment yields to rill lengths of 0.5 to 8 m. A computational method was developed to estimate, with a given error, the sampling length at which sediment yield reached transport capacity, with the regressed functional relation of sediment load to rill length, which determined with experimental data obtained. With the suggested algorithm, sample lengths of rill on different slopes and at different flow rates were estimated with the related functional relation, as determined with the experiments. Further more, the influences of slope and flow rate on sample length were presented.