Abstract: Recently,the duckbill-shaped elastomer check valves have installed on many oceanic outfalls and diffusers to prevent sea water and sediment intrusion.In this paper,the mixing characteristics of a duckbill valve jet has been studied by using acoustic Doppler velocimeter(ADV) and laser-induced fluorescence(LIF) techniques.The measurement results reveal a complicated near-field flow extending up to about 12.5 jet diameters,within which the turbulence intensity is much higher than that of a round jet,the jet spreading in the major and minor axes are distinctly different,and an axis-switching is observed in this region.Beyond.the near-field,however,the jet mixing characteristics are similar to an equivalent round jet.