An attempt was made to use gamma ray transmission method to measure sediment concentration in runoff. Two types of soils (silt loess and clay loess), three increments of sediment concentration (3%, 5% and 10%) and 3 sampling periods (6 s, 10 s, 20 s) were adopted for sediment measuring experiments, so as to validate the feasibilities of measuring sediment concentration with gamma ray method and to determine the response of measured sediment concentration to different conditions. The theoretical algorithm was derived for determining sediment concentration with gamma ray method. A simple relation of sediment concentration with gamma ray readings was statistically given. Experimental data were used to verify the correctness of the theoretical equation and to check the consistency of the theoretical results with those from experiments. Results showed very good linear relationship between sediment concentrations and gamma ray readings, as well as very good consistency between measured sediment concentrations and the theoretically computed values. The gamma ray method can be used to determine sediment concentrations steadily, reliablly and precisely.