Abstract: The flow fields of a vertical round jet in regular and random waves are measured by a 3-D Acoustic-Doppler Velocimeter. The phenomenon of "twin peaks" is found on the cross-sections of the jet vertical velocity in regular waves when close to the free surface, but it is less obvious on the cross-sections of the jet in random waves. Under the principle of equivalent wave energy and wave energy flux, the jet flow fields in regular and random waves are quantitatively compared. The results show that the jet centerline velocity exhibits a faster decay in regular waves than that in random waves, which could be directly related to the phenomenon of "twin peaks".