Using the new gas tracer technique, the stream reaeration coefficient was measured in the field. Ethylene (C2H4) is used as a tracer gas and Rhodamine WT as a dispersion and advection tracer. After the step type injection of both tracers, water samples are taken from three recoveries downstream. 　The temporal distribution of the tracers are compared and analyzed for the decay rate of the gas tracer. The volatilization rate of ethylene in the flow is estimated by the peak method and the mass method. The mean velocity, the mean depth and the longitudinal dispersion coefficient are also calculated by the results of the dye study. Estimated values of the hydraulic parameters and the reaeration coefficient are within a reasonable range and show good agreement with the predicted values through the theoretical formula or the semi-empirical one. 　The gas tracer technique proves to be very useful for the study of the volatilization of CO2 or pesticides which will tend to be accumulated in the coastal sea.