Abstract: Method and technical problems in measuring the concentration of entrained air in the high-velocity and sediment-laden flow during model test by utilizing the γ-ray absorption principle are described. By preliminary analysis, it is found that the errors resulting from the measurement of entrained air by using the method of radioassay are less than±0.25％. Satisfactory results have been obtained in practical applications. The method is non-contacting and will not disturb the flow conditions. The measurement can be done rapidly with reliability.